cleanup of prettyPrint

- moving to syntax.js
- adding to case macros so you can call prettyPrint during macro expansion
- adding --step-hygiene flag so you can see hygienic renames when stepping
adding pretty printer to syntax
build
fixing hygiene bug in withSyntax #161

withSyntax needs to create its own mark instead of reusing the calling
macro’s mark
build
Merge branch 'master' of https://github.com/mozilla/sweet.js into infix
Refactor syntaxCase for lookbehind matching.
build for patterns.matchLookbehind