interner just uses uints, not idents with syntax context

also, move interner fns out of parser_testing
remove unused cx's
rand example fixup
fixing interner function names
remove interner from tt_reader
remove interner field from string_reader
removed unused imports (and one unused argument)
remove unused get_ident_interner's
working on let-hygiene
added fresh-name fn
moved TLS of sctable to ast_util, hid parameter in hygiene calls
starting to add sexp encoder
tmp
tmp
tmp -- but compiles!
tmp
tmp
tmp
expansion now covers macros, test cases for renaming
removed unneccessary SyntaxExpander structs