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)
detached
Status Job ID Name Coverage
  Build
failed #1147796
build: [4.12.0]

00:13:35

 
Name Stage Failure
failed
build: [4.12.0] Build
$ sudo apt-get update
ERROR: ld.so: object 'libdl.so.2' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Err:1 http://deb.debian.org/debian testing InRelease
400 Bad Request [IP: 199.232.178.132 80]
Reading package lists...
E: Failed to fetch http://deb.debian.org/debian/dists/testing/InRelease 400 Bad Request [IP: 199.232.178.132 80]
E: The repository 'http://deb.debian.org/debian testing InRelease' is no longer signed.
Cleaning up file based variables
ERROR: Job failed: exit code 1