Add ability to match more terms in MacroContext::expand

You can now match against more grammar productions with the MacroContext::expand method:

syntax m = ctx => {
  let expr = ctx.expand('BinaryExpression');
  // ...
}

syntax n = ctx => {
  let ident = ctx.expand('IdentifierExpression');
  // ...
}

m foo() + 100 - [1, 2, 3]   // <- everything is matched
n foo() + 100 - [1, 2, 3]   // <- only `foo` is matched