FIFO with simultaneous `push` and `pop` when full

[phsauter]

Paul and Thomas would like a fifo that can do that.
This will likely require another way to cut the IO paths for timing reasons.

Paul and Thomas cannot be bothered to report this properly so I am doing it.

Tangentially related to #264

[phsauter]

A FIFO with different cut paths for different timing cases might still be interesting in the future but for now closed with #322