Currently, AsyncFIFO fails if you specify a depth that isn't a power of 2, and AsyncFIFOBuffered fails if you specify a depth that isn't a power of 2 plus one. It is rarely useful to request a FIFO that must be the exact size, down to one element, and this can be annoying.

I propose a new argument for the AsyncFIFO/AsyncFIFOBuffered constructor, exact_depth. When True, the current behavior is preserved. When False (the new default), the FIFO will use the smallest available depth that is not less than the requested one.