macro_rules: proper FIRST/FOLLOW computations for checking macro_rule…

…s validity.

See RFC amendment 1384 and tracking issue 30450:
  rust-lang/rfcs#1384
  rust-lang#30450

Moved old check_matcher code into check_matcher_old

combined the two checks to enable a warning cycle (where we will
continue to error if the two checks agree to reject, accept if the new
check says accept, and warn if the old check accepts but the new check
rejects).