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

Apparently it can't live in the main binary, since on non-Linux
platforms, dynamics libs won't find symbols in the binary. This
removes the crate_map pointer from rust_crate again, and instead
passes it as an extra argument to rust_start. Rustboot doesn't pass
this argument, but supposedly that's okay as long as we don't actually
use it on that platform.

This overloads the meaning of RUST_LOG to also allow
'module.submodule' or 'module.somethingelse=2' forms. The first turn
on all logging for a module (loglevel 3), the second sets its loglevel
to 2. Log levels are:

 0: Show only errors
 1: Errors and warnings
 2: Errors, warnings, and notes
 3: Everything, including debug logging

Right now, since we only have one 'log' operation, everything happens
at level 1 (warning), so the only meaningful thing that can be done
with the new RUST_LOG support is disable logging (=0) for some
modules.

TODOS:
 * Language support for logging at a specific level
 * Also add a log level field to tasks, query the current task as well
   as the current module before logging (log if one of them allows it)
 * Revise the C logging API to conform to this set-up (globals for
   per-module log level, query the task level before logging, stop
   using a global mask)

Implementation notes:

Crates now contain two extra data structures. A 'module map' that
contains names and pointers to the module-log-level globals for each
module in the crate that logs, and a 'crate map' that points at the
crate's module map, as well as at the crate maps of all external
crates it depends on. These are walked by the runtime (in
rust_crate.cpp) to set the currect log levels based on RUST_LOG.

These module log globals are allocated as-needed whenever a log
expression is encountered, and their location is hard-coded into the
logging code, which compares the current level to the log statement's
level, and skips over all logging code when it is lower.

This reduces crate_ctxt copying (which unfortunately did not
result in a noticeable speedup), and makes the role of that
type a bit more clear.

rustc: Remove the "boxed" check, and make the static_size_of_tag recursion-eliminating transformation deep