Port to RFC 61 streams

[whitequark]

It would be nice if LUNA used RFC 61 streams internally. (Also, records have been removed in Amaranth 0.6, so the options are RFC 61 streams, copying Record from Amaranth 0.5 sources, or designing an entirely new incompatible interface.)

[martinling]

Some related discussion at #121.

[whitequark]

There is a work-in-progress stream library called Katsuo Stream that incorporates some ideas we've had. It may not be what amaranth.lib.stream eventually becomes, but it's guaranteed to be compatible with RFC 61 streams and fairly likely to be compatible with future standard stream extensions.

An important aspect of it is that we propose a payload with four members:

• data
• first (optional)
• last (optional)
• end (optional)

In this proposal, first indicates the beginning of a packet (and can be combined with data), last the ending of a packet (and can be combined with data), while end causes all other fields to be ignored and is used to mean "we didn't know exactly when the packet ends before, but we do know it's ended now". I believe this will resolve #121.