GAP: add and use ReplaceBinding helper

[fingolfin]

This is a variant of Oscar's ReplaceGapFunc but I figure it is
be useful for other code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.47%. Comparing base (1992296) to head (c09bcf2).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1085      +/-   ##
==========================================
+ Coverage   75.43%   75.47%   +0.03%     
==========================================
  Files          55       55              
  Lines        4580     4587       +7     
==========================================
+ Hits         3455     3462       +7     
  Misses       1125     1125              

Files with missing lines	Coverage Δ
gap/exec.g	100.00% <100.00%> (ø)
gap/systemfile.g	100.00% <100.00%> (ø)

[lgoettgens]

no idea about these low-lever GAP functions, but seems to work

[ThomasBreuer]

Yes, why not.

If the variable in question has no value (yet) then VAL_GVAR will run into an error, which is fine.