Store items and decls in vecs to preserve input order, index externally. Implement block-local name lookup.

Encode and decode tag types in dwarf properly. Add list module to std. Shift rustc to use std.util.option. Fix various dependent bugs. Closes #73.

Bind pattern slots with ?, drop parens from 0-ary tag constructors, translate 0-ary constructors as constants. Rustc loses ~300kb.

Switch tags to purely nominal, removing TY_iso and TY_idx. Seems to mostly work, possibly a little bumpy. Changes a lot.

Reinstate commit 9f0eaa65 with sufficient fixes to hopefully not break tinderboxes.

Revert "More work on lexer.rs: start using keyword hashtable, handle more lexemes." due to tinderbox bustage

This reverts commit 9f0eaa65.