Auto merge of #37701 - Mark-Simulacrum:macro-parser-impvement, r=jseyfried

Auto merge of #37701 - Mark-Simulacrum:macro-parser-impvement, r=jseyfried

Macro parser performance improvements and refactoring

This PR locally increased performance of https://github.com/rust-lang/rust/issues/37074 by ~6.6 minutes.

Follow up to https://github.com/rust-lang/rust/pull/37569, but doesn't focus explicitly on expansion performance.

History is relatively clean, but I can/will do some more polishing if this is deemed mergeable. Partially posting this now so I can get Travis to run tests for me.

r? @jseyfried