Add evmap

A generalized heap-like monoid structure, ev means "evidence".

It is used heavily in my implementation of treiber's stack and flat combiner. And it looks like gps side is also using similar structure. Rust side has a gmap loc (frac * lock_state * dec_agree val).

Now, the lowest layer is for any proper K A Q: gmapUR K (prod Q (dec_agreeR A)). The next layer instantiated Q with fracR.