Introduce --stats

[mtzguido]

This adds a --stats option to print out the time spent in several places of the compiler (we should add more). It displays the number of calls to a given "key", the total time taken by the calls, the time taken by the calls ignoring subcalls, and the time spent before raising an exception.
Example for the report in #3800:

$ ./bin/fstar.exe Bug3800.fst --stats --admit_smt_queries true
Verified module: Bug3800
All verification conditions discharged successfully
Stats:
                   key     calls      tree     point       exn
             norm_term   1471862  28148 ms  28148 ms      0 ms
           tc_one_file         8  40820 ms   8011 ms      0 ms
             norm_comp         4   4658 ms   4658 ms      0 ms
    Parser.Dep.collect         1      4 ms      3 ms      0 ms
                 parse         2      0 ms      0 ms      0 ms
           simplify_vc      1290  21075 ms      0 ms      0 ms
         desugar_decls         1      0 ms      0 ms      0 ms
      hash_dependences         7      0 ms      0 ms      0 ms
      deep_compress_se        15      0 ms      0 ms      0 ms

So out of 40 seconds it takes to check the file, 28 are spent in normalizing terms. 21s are spent under simplify_vc, but they are completely accounted by measured subcalls (in this case normalization).

There is support for profiling already. I wrote this since I wanted an easy way to see where time is being spent without having to look up the profiling tags. Also, --profile requires a module name, which is not really useful. We could coallesce profiling into these new stats?

Also, uncommenting this line

let profile  (f: unit -> 'a) (module_name:option string) (cid:string) : 'a =
  //Stats.record cid fun () ->

will cause all profiling events to be recorded as stats. And that makes the example above look something like:

Stats:
                                                          key     calls      tree     point       exn
  FStarC.TypeChecker.Normalize.normalize_with_primitive_steps   1471862  29453 ms  29453 ms      0 ms
                  FStarC.TypeChecker.Normalize.normalize_comp         4   4663 ms   4663 ms      0 ms
                             FStarC.TypeChecker.Tc.encode_sig        15  10016 ms   4149 ms      0 ms
                        FStarC.Universal.tc_source_file.check         1  44139 ms   4092 ms      0 ms
           FStarC.SMTEncoding.EncodeTerm.normalize_refinement   1441772   6146 ms    601 ms      0 ms
                                                    norm_term   1471862  30037 ms    583 ms      0 ms
                       FStarC.TypeChecker.Normalize.normalize   1471862  30603 ms    566 ms      0 ms
                                FStarC.Universal.extend_tcenv         7     19 ms     19 ms      0 ms
                                          FStarC.CheckedFiles         8     18 ms     18 ms      0 ms
                                  FStarC.TypeChecker.Rel.whnf     24326     48 ms      9 ms      0 ms
        FStarC.TypeChecker.Rel.try_solve_deferred_constraints       320     40 ms      5 ms      0 ms
                   FStarC.TypeChecker.Tc.tc_sig_let-tc-phase1         1  17751 ms      4 ms      0 ms
                       FStarC.TypeChecker.Rel.check_subtyping      1096     23 ms      4 ms      0 ms
                                           Parser.Dep.collect         1      3 ms      2 ms      0 ms
                   FStarC.TypeChecker.Tc.tc_sig_let-tc-phase2         1   8520 ms      1 ms      0 ms
                                   FStarC.Parser.Dep.discover         1      1 ms      0 ms      0 ms
                                                        parse         2      0 ms      0 ms      0 ms
                  FStarC.TypeChecker.Rel.normalize_refinement       186      0 ms      0 ms      0 ms
                           FStarC.TypeChecker.Util.generalize         2      0 ms      0 ms      0 ms
                       FStarC.TypeChecker.Tc.process_one_decl        15  36291 ms      0 ms      0 ms
                           FStarC.TypeChecker.Rel.simplify_vc      1290  21534 ms      0 ms      0 ms
                                                desugar_decls         1      0 ms      0 ms      0 ms
                                                  simplify_vc      1290  21534 ms      0 ms      0 ms
        FStarC.TypeChecker.Rel.solve_deferred_to_tactic_goals       318      0 ms      0 ms      0 ms
                               FStarC.TypeChecker.Rel.try_teq        32      0 ms      0 ms      0 ms
                                             hash_dependences         7      0 ms      0 ms      0 ms
                                             deep_compress_se        15      0 ms      0 ms      0 ms
                                                  tc_one_file         8  44178 ms      0 ms      0 ms
                              FStarC.TypeChecker.Rel.sub_comp         2  21530 ms      0 ms      0 ms
                FStarC.SMTEncoding.EncodeTerm.norm_with_steps        76      0 ms      0 ms      0 ms
                                    FStarC.TypeChecker.Rel.sn        66      0 ms      0 ms      0 ms
                        FStarC.Universal.tc_source_file.parse         1      0 ms      0 ms      0 ms
                     FStarC.TypeChecker.Rel.norm_with_steps.8        66      0 ms      0 ms      0 ms
               FStarC.SMTEncoding.Encode.norm_before_encoding        32   3475 ms      0 ms      0 ms
                 FStarC.Parser.Dep.topological_dependences_of         1      0 ms      0 ms      0 ms
                             FStarC.TypeChecker.Tc.tc_sig_let         1  26271 ms      0 ms      0 ms
                                                    norm_comp         4   4663 ms      0 ms      0 ms

(see also the 10s in encode_sig!)

[mtzguido]

Updated to take a boolean (--stats true/--stats false) so it can be enabled and disabled, and even pushed/popped within a file. Stats are only counted while the option is enabled, and reported before exit if they were ever enabled.