CMRAs with partial cores.

Based on an idea and WIP commits of J-H. Jourdan: the core of a CMRA
A is now a partial function A → option A.

TODO: define sum CMRA
TODO: remove one shot CMRA and define it in terms of sum
1 job for master
Status Job ID Name Coverage
  Test
passed #413
coq
buildjob

00:03:35