add hygiene support fns, move them around.

also adds test cases
flip the switch on let renaming
renaming test cases
comments in ast.rs
removed unneccessary SyntaxExpander structs
separate ItemDecorator from ItemDecorator
refactor so tt_fold only requires an ident->ident fn
adding test case to check marking/unmarking
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added parse_token_trees.rs
starting to add sexp encoder