Fix #6696
change some uses of core to std

specifically, all uses of core in libsyntax except for the 'use core::prelude::*' ones
add renaming and sctable funs
change to newer macro escape mechanism, using uints in more places
comments & whitespace
parser comments
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
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