starting to add sexp encoder
parser comments
working on let-hygiene
removed obsolete reference to purity
rename repr to name
just use TLS interner
removed some interner fields
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
added fresh-name fn
remove interner from tt_reader
remove interner field from string_reader
removed unused imports (and one unused argument)
remove unused get_ident_interner's
moved TLS of sctable to ast_util, hid parameter in hygiene calls
tmp