Added nonterminal_subscript_int

We can now fetch elements from a concrete list (even works when there is
a surrounding non-terminal);

Given 

syntax A = a: "a";

syntax As = as: {A ","}*;

then ([As] "a,a")[0] will return the first "a".
Merge branch 'master' of https://github.com/cwi-swat/rascal.git