Switch to using JLLs

[fingolfin]

Note that this means we'll load an extra copy of the GMP for now. If this turns out to be a problem, I could drop the GMP_jll bit again.

This is not done; in particular, I am experimenting more with how to launch the GAP inside GAP.jl. Right now I am experimenting with just launching it via julia: In particular, instead of $(julia -e 'import GAP; print(GAP.gap_exe())') one can do import GAP; GAP.Globals.SESSION() to get a GAP prompt. This is missing a banner (easy to fix) and also doesn't handle command line arguments (slightly more work to fix, but doable)

Fixes #415

[codecov]

Codecov Report

Merging #421 into master will increase coverage by 26.04%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           master     #421       +/-   ##
===========================================
+ Coverage   38.11%   64.15%   +26.04%     
===========================================
  Files         214       68      -146     
  Lines       31589     5343    -26246     
===========================================
- Hits        12039     3428     -8611     
+ Misses      19550     1915    -17635     

Impacted Files	Coverage Δ
deps/build.jl	60.00% <66.66%> (+60.00%)	⬆️
src/GAP.jl	72.17% <100.00%> (-0.24%)	⬇️
docs/make.jl	0.00% <0.00%> (-84.62%)	⬇️
pkg/JuliaInterface/src/calls.c	21.79% <0.00%> (-56.60%)	⬇️
pkg/JuliaInterface/src/convert.c	86.48% <0.00%> (-0.70%)	⬇️
pkg/JuliaInterface/src/calls.h	0.00% <0.00%> (ø)
.../gap-4.11.0-julia-1.3/pkg/profiling-2.3/src/md5.cc
....11.0-julia-1.4/pkg/profiling-2.3/src/profiling.cc
...filing-2.3/src/gap_cpp_headers/gap_cpp_mapping.hpp
...-4.11.0-julia-1.5/pkg/profiling-2.3/doc/manual.six
... and 144 more

[mohamed-barakat]

Works for me, however, I would have prefered to see the GAP banner, simply to see which version of GAP I am about to run:

$ $(julia -e 'import GAP; print(GAP.gap_exe())')
gap> 1+1;
2

[mohamed-barakat]

This is really nice:

import GAP; GAP.Globals.SESSION()

Is it possible to assign GAP.Globals.SESSION to an exported function called gap or startgap? I would suggest following the sage convention here:

using GAP
gap()
gap>

[fingolfin]

That hypothetical gap() function should do more things, too; e.g. it could turn the break loop back on temporarily