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 13 minutes and 35 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1147796
build: [4.12.0]


Name Stage Failure
build: [4.12.0] Build
$ sudo apt-get update
ERROR: object '' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Err:1 testing InRelease
400 Bad Request [IP: 80]
Reading package lists...
E: Failed to fetch 400 Bad Request [IP: 80]
E: The repository ' testing InRelease' is no longer signed.
Cleaning up file based variables
ERROR: Job failed: exit code 1