add Fifo.length function

[fredcy]

My app frequently wants to know the length of the Fifo (for display to the user) and the Fifo can grow to be thousands of items long, so I want a fast way to obtain that length value. I could use Fifo.toList |> List.length but that involves both list reversal (within toList) and traversal again (in List.length) on each call.

So this change has the Fifo keep its current length as part of the state. This does mean additional work on every insert, remove, and toList call to recalculate that length. I have not benchmarked any times before and after this change.

If this PR is unacceptable, please let me know and I'll probably publish my forked version as a new package.