starting to add sexp encoder
add hygiene support fns, move them around.
fix a bunch of bugs, move renaming up to the block traversal level

also adds test cases
expansion now covers macros, test cases for renaming
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