[imcc] fix subid quote stripping

still a battlefield are pmc_const's
which are unquoted, unescaped possibly twice,
and then converted to pmc values.
[imcc] add mk_string, unify string quote and unescape handling

Also for mk_sub_address_fromc, _mk_adress, do_loadlib, .lex, mk_const_ident.
This broke the string GC somewhow.