Reduce backtracking in Lua “for” statement parsing.
Reduce backtracking in Lua “for” statement parsing.

When parsing “for a in …” the parser will currently first try to match a
rule of the form “for a = …” and then backtrack, causing the name “a” to
be parsed twice. This refactoring delays the choice and avoids backtracking.

This partly addresses the discussion in #32.

ColinH/PEGTL,ColinH/PEGTL