set() can now reference other named token sets

[superbobry]

I originally wanted this feature to use new syntax, e.g.

%generate foo = set('a' | 'b');
%generate bar = foo | set('c');
%generate boo = foo & set('c');

so that

• an identifier in set() is always a terminal/nonterminal;
• identifier outside of set is always another named token set.

However, that turned out to be a bigger change than what I was hoping for. So, this commit allows identifiers in set() to reference both terminals/nonterminals and other named token sets. Rewriting the above example,

%generate foo = set('a' | 'b');
%generate bar = set(foo | 'c');
%generate boo = set(bar & 'c');