field renaming
commenting parser
change to parsing using tts

also, updates test cases a bit
auto merge of #5559 : jbclements/rust/change-to-tt-based-parsing, r=jbclements

Changes the parser to parse all streams into token-trees before hitting the parser proper, in preparation for hygiene.  As an added bonus, it appears to speed up the parser (albeit by a totally imperceptible 1%).

Also, many comments in the parser.
Also, field renaming in token-trees (readme->forest, cur->stack).