Tydi TODO

The following things have come up that are either broken or aren't very nice about the current specification.

- Asynchronous signals cannot be reversed (#23). Proposed solutions:
- Make reversal part of fields instead of Stream nodes. (This is problematic because at best it introduces implicit streams again. It also means tools cannot reasonably place registers/clock domain crossings automatically.)
- Remove the signals from the specification, but add a separate type system (based on Tydi, bit supporting reversal) for user-defined signals.
- Keep the current specification for signals that tools may insert registers into, but add a separate type system (based on Tydi) for user-defined signals that supports reversal that the tools don't do anything with.
- Order of asynchronous signals and streams is not maintained.
- The user-friendly representation notes should be removed. This and any requirements on field names beside uniqueness are entirely in user control. Code generators are considered to be users in this context; this means that generators may impose additional constraints.
- Add a flag to `Stream` nodes and to physical streams that specifies whether empty sequences are supported or not.
- Maybe add a note that `Tuple(N, T)` may be used as a shorthand for `Group(0: T, 1: T, ..., N-1: T)`.