When parsing rules gyp does not complain about undefined string identifiers.

For example, it does not error when parsing this rule:

rule a { condition: $x }

With these changes it now walks the condition AST and makes sure all string
references are valid, including wildcard expansion.

While I'm here I uncovered a slight error with the Children() method on the Of
node, where it would always set the Strings to nil if there were none, which is
now fixed.

I also discovered some existing tests that were broken by this (they did not
have a $foo4 string defined) so I "fixed" those too.

Switch to tracking strings in the lexer struct and checking for existence of
strings as they are produced by the parser.