Get rid of GAPTypes.jl

[fingolfin]

This requires a custom GAP branch for now, but see also gap-system/gap#3867

Fixes #305
Fixes #310

[codecov]

Codecov Report

Merging #322 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #322   +/-   ##
=======================================
  Coverage   76.58%   76.58%           
=======================================
  Files          62       62           
  Lines        4428     4428           
=======================================
  Hits         3391     3391           
  Misses       1037     1037

Impacted Files	Coverage Δ
src/GAP.jl	92.85% <ø> (ø)	⬆️
deps/build.jl	0% <ø> (ø)	⬆️

[fingolfin]

Unfortunately the new GAP.jl in this PR requires the new GAP; mixing either of them with an old version of their counterpart doesn't work (some cases work, but not the general thing).
I see two options to go forward:

1. We just accept that, and try to merge both changes about the same time, and then immediately make a new GAP.jl release so as to not annoy users too much
2. I try to come up with a way to split this into separate updates which always retain compatibility to one version prior:

• this PR would get split into at least two parts: one adds the call to GAP_register_GapObj but basically nothing else (in particular, doesn't drop GAPTypes)
• we make a release of GAP.jl immediately afterwards
• next, GAP is update by merging julia: remove the need for GAPTypes.jl gap-system/gap#3867 into master and stable-4.11; this will work with the latest released GAP.jl; it drops use of GAPTypes from GAP
• finally, we update GAP.jl once more, removing references to GAPTypes here, too.

The second approach is more work but should be safer, and requires less perfect timing. So I'll try and see if I can do it this way.

[kalmarek]

I'd go for the first version, it's less work and GAP.jl is still living on the bleeding edge ;-)

[fingolfin]

@kalmarek not sure what you refer to with "first version"? Anyway, this PR is already merged