Make CI faster especially on PR builds

[KtorZ]

Issue Number

#242

Overview

- Made PR builds operating slightly differently than the "standard build".
  The idea is to only do what we really need to do for PRs and keep the
  more expensive operations on merges or cron jobs.

- Prune a bit some of the `.stack` internals in order to reduce the size
  of the cached directory. This should speed up a bit the cache
  downloads and uploads of each job.

- Remove the 'rust' language to avoid having to setup rust on each job.
  In the end, we only need rust in order to install rust binaries, after
  what, we only need the statically linked binaries.

- Split the compilation step into three jobs stages:
  - rust stuff
  - stackage snapshot
  - haskell project (only source and bench, tests are compiled in their
    corresponding test jobs)

Comments

[KtorZ]

Plan is to bypass CI here since this is actually updating the whole CI and fixing quite a bunch of issues we have these days.