Document FIFO properties

[rroohhh]

It would be nice if various properties of the FIFOs would be documented. What comes to mind is

• maximum throughput (SyncFIFO of depth == 1 only does a throughput of 50%, same for SyncFIFOBuffered with depth <=2)
• dependencies between the interface signals (does w_stream.ready depend combinatorially on r_stream.ready?, does r_stream.valid depend combinatorially on w_stream.valid?)

[whitequark]

We could do that, but I think I'm more interested in deprecating lib.fifo in favor of a stream-based lib.queue with nicer properties (e.g. higher throughput in edge cases).