Add stream support for isochronous endpoints.

[antoinevg]

This PR depends on #278

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• Adds stream support to both IN and OUT isochronous endpoints.
• Uses amaranth.lib.stream.Interface for the implementation instead of luna.stream.StreamInterface.
• Introduces a new data type luna.stream.future.Packet that adds first and last signals to an Amaranth stream payload in order to support packetization.

[mndza]

Why are USBIsochronousStreamInEndpointand USBIsochronousStreamOutEndpoint implemented in different files? This is a bit inconsistent with the rest of the code.

[antoinevg]

I'd dearly love to split up the rest of the code as well, having everything in single files makes it really hard to both find and work on things.

I'm a firm believer in the "one big thing per file" principle unless it's a bunch of small things that are closely related like type definitions.

Doing a PR that does the same for the other files in luna/gateware/usb/usb2/endpoints/ should have minimal downstream impact as these are all re-exported via the top-level luna.usb2 module path, e.g.

luna/examples/usb/isochronous_count.py

Line 15 in a71b891

from luna.usb2 import USBDevice, USBIsochronousInEndpoint

That said, I am open to changing this for now if we want to leave this discussion to a later time!

[mndza]

Looking great 😄