Summary says it all. Actually, only nested objects and functions
are handled, but that's better than before. The fold that I was using
before to traverse a crate wasn't working correctly, because annotations
have to reflect the number of local variables of the nearest enclosing
function (in turn, because annotations are represented as bit vectors).
The fold was traversing the AST in the wrong order, first filling in
the annotations correctly, but then re-traversing them with the bit
vector length for any outer nested functions, and so on.

Remedying this required writing a lot of tedious boilerplate code
because I scrapped the idea of using a fold altogether.

I also made typestate_check handle unary, field, alt, and fail.

Also, some miscellaneous changes:
* added annotations to blocks in typeck
* fix pprust so it can handle spawn
* added more logging functions in util.common
* fixed _vec.or
* added maybe and from_maybe in option
* removed fold_block field from ast_fold, since it was never used

Also did some refactoring in typestate_check. All test cases in
compile-fail that involve uninitialized vars now fail correctly!
(All eight of them, that is.)

Also changed the ts_ann field on statements to be an ann instead,
which explains most of the changes.

As well, got rid of the "warning: no type for expression" error
by filling in annotations for local decls in typeck (not sure whether
this was my fault or not).

Finally, in bitv, added a clone() function to copy a bit vector,
and fixed is_true, is_false, and to_str to not be nonsense.

This makes passing them around cheaper. There is now a table (see
front/codemap.rs) that is needed to transform such an uint into an
actual filename/line/col location.

Also cleans up the span building in the parser a bit.

The typestate checker (if it's uncommented) now correctly rejects a
trivial example program that has an uninitialized variable.

This reverts commit 846f2e2b.

Switch all vases of vec += elt to vec += vec. Prohibit former in rustboot. Tweak std lib vec fns in process.

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.