lib.fifo: round up AsyncFIFO{,Buffered} depth to lowest valid value.

lib.fifo: round up AsyncFIFO{,Buffered} depth to lowest valid value.

Unless exact_depth=True is specified.

The logic introduced in this commit is idempotent: that is, if one
uses the depth of one AsyncFIFOBuffered in the constructor of another
AsyncFIFOBuffered, they will end up with the same depth. More naive
logic would result in an unbounded, quadratic growth with each such
step.

Fixes #219.