Replace GAP's ExecuteProcess by Julia code

[fingolfin]

This is a step towards getting rid of the GAP SIGCHLD handler

I only did the most basic test (run Exec("ls") from a GAP prompt()), so more tests are absolutely missing, also using this with packages.

On the other hand, things seem to work surprisingly well even without this patch. But SIGCHLD handling in GAP is something we really should try to get rid of (CC @benlorenz)

Resolve #240

[benlorenz]

Thanks for working on this, I can try to run some tests with polymake by next week, also with my own patches to polymake removed.

The latest polymake_jll version 400.500.0 also has an updated patch for the sigchild issues which will temporarily set the the SIGCHLD handler to the default while polymake is running subprocesses. I had already patched this for some wrapper compilation issues in a different way but hit such errors again while experimenting with the 4ti2 wrappers. With my new patch all tests look good so far.

[codecov]

Codecov Report

Merging #726 (44d4081) into master (4ab5336) will decrease coverage by 0.02%.
The diff coverage is 75.60%.

❗ Current head 44d4081 differs from pull request most recent head 9af6091. Consider uploading reports for the commit 9af6091 to get more accurate results

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   78.07%   78.04%   -0.03%     
==========================================
  Files          46       48       +2     
  Lines        3649     3690      +41     
==========================================
+ Hits         2849     2880      +31     
- Misses        800      810      +10     

Impacted Files	Coverage Δ
src/exec.jl	70.96% <70.96%> (ø)
src/GAP.jl	82.24% <83.33%> (+0.06%)	⬆️
gap/exec.g	100.00% <100.00%> (ø)