Merge #326

326: Syntax: Rewrite macro_rules r=ehuss a=ehuss

macro_rules failed in a lot of cases, this reworks it to fully support:
- Complete matcher syntax.
- Better bracket balancing support.
- Add `lifetime` designator (1.28).
- More resilient to odd syntax within the transcriber.  The content doesn't have
  to be Rust syntax, so it can sometimes get confused. Add a few escapes so it
  at least doesn't barf.
- At-most-once kleene operator (2018).

Closes #311 

<img width="903" alt="image" src="https://user-images.githubusercontent.com/43198/43736311-018cc660-9972-11e8-8931-d4d2bd5e7501.png">


Co-authored-by: Eric Huss <[email protected]>

Author: bors[bot]