Solver: basically get rid of continuations

By adding a CMap constructor to our raw constraints, we can implement
map directly within the constraint language, so we don't need to
define ('a co) as a pair of a constraint and a continuation anymore,
raw constraints are expressive enough.
1 job for !16 with purer-constraints in 18 minutes and 4 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
passed #1153765
build: [4.12.0]