feaLib.parser is too builder-specific

[simoncozens]

I think this is syntactically (but not semantically) valid feature code:

feature foo {
  lookup bar;
} foo;

I would expect a parser to be able to turn that into an abstract syntax tree that you can manipulate, and then a builder to later complain that bar is not defined. However, trying to run Parser("foo.fea").parse() on this feature code fails because the parser cannot resolve the lookup.