@gap and g_str macro call evalstr at parsetime instead of runtime, which is surprising

[fingolfin]

As a result, we have this:

julia> f(x)  = @gap (1,2)(3,x)
ERROR: LoadError: Error thrown by GAP: Error, Variable: 'x' must have a value
not in any function at stream:1

while this works

julia> f(x)  = GAP.evalstr("(1,2)(3,$x)")
f (generic function with 1 method)

julia> f(4)
GAP: (1,2)(3,4)

The following options come to my mind:

1. keep this and just document it (but seems pretty useless)
2. fix it by returning a properly quoted code snippet
3. remove one or both offending macros