add hygiene support functions
added fresh-name fn
moved TLS of sctable to ast_util, hid parameter in hygiene calls
starting to add sexp encoder
add hygiene support fns, move them around.
tmp -- but compiles!
tmp
tmp
tmp
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