Make the lexer use a uniquely linked stack

[catamorphism]

As per FIXME on syntax:parse::lexer::tt_frame: figure out how to have a uniquely linked stack and change @ to ~

[nikomatsakis]

It is unclear to me why this should be important. I'm going to open a PR to remove the fixme (which is now located in src/libsyntax/ext/tt/transcribe.rs).