Optimize GAP function calls

[fingolfin]

Before:

julia> g=GAP.evalstr("(1,2)"); func = GAP.Globals.POW;

julia> @btime func(2,g);
  111.728 ns (3 allocations: 64 bytes)

After:

julia> g=GAP.evalstr("(1,2)"); func = GAP.Globals.POW;

julia> @btime func(2,g);
  26.508 ns (0 allocations: 0 bytes)

[codecov]

Codecov Report

Merging #948 (a2ad5b0) into master (5aa8c47) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #948      +/-   ##
==========================================
- Coverage   75.68%   75.52%   -0.16%     
==========================================
  Files          51       51              
  Lines        4166     4168       +2     
==========================================
- Hits         3153     3148       -5     
- Misses       1013     1020       +7     

Files	Coverage Δ
src/ccalls.jl	99.19% <100.00%> (+0.01%)	⬆️

... and 4 files with indirect coverage changes