This was supposed to be in a previous commit. I don't know what happened.

Enable typestate checking (just for uninitialized vars) and un-XFAIL the
relevant tests for stage0.

I changed instantiate to print out a more helpful error message,
which required passing it a session argument. To avoid
threading extra arguments through a lot of functions,
I added a session field to ty_ctxt.

In rustc, nested patterns were potentially matching when they shouldn't
match, because a loop index wasn't being incremented. Fixed it and added
one test case.

I changed the error message to also suggest checking the -L flag
when this happens.

The code for taking pattern-bound variables was being interspersed
with pattern code, so that if a nested pattern failed partway through,
a variable would be taken but never dropped (because the drop code
is inside the block representing the action for the pattern). For
example, in the pattern foo(?i, bar(some[t](_)), _), if the scrutinee
was foo(x, bar(none[t]), y), the variable i would be taken but never
dropped. The patch fixes this bug.

Fix bug in handling of expr_alt (postcond for alts was being intersected with postcond for scrutinee)

(needed for typestate_check).

Also changed a (log; fail) to (log_err; fail) in typestate_check,
and added some more logging funs in util.common.

Added support for self_method, cont, chan, port, recv, send, be,
do_while, spawn, and ext; handled break and cont correctly.
(However, there are no non-xfailed test cases for ext or spawn in
stage0 currently.)

Although the standard library compiles and all test cases pass with
typestate enabled, I left typestate checking disabled as rustc
terminates abnormally when building the standard library if so,
even though it does generate code correctly.

Lots of work on typestate_check, seems to get a lot of the way
through checking the standard library.

* Added for, for_each, assign_op, bind, cast, put, check, break,
and cont. (I'm not sure break and cont are actually handled correctly.)

* Fixed side-effect bug in seq_preconds so that unioning the
preconditions of a sequence of statements or expressions
is handled correctly.

* Pass poststate correctly through a stmt_decl.

* Handle expr_ret and expr_fail properly (after execution of a ret
or fail, everything is true -- this is needed to handle ifs and alts
where one branch is a ret or fail)

* Fixed bug in set_prestate_ann where a thing that needed to be
mutated wasn't getting passed as an alias

* Fixed bug in how expr_alt was treated (zero is not the identity
for intersect, who knew, right?)

* Update logging to reflect log_err vs. log

* Fixed find_locals so as to return all local decls and exclude
function arguments.

* Make union_postconds work on an empty vector (needed to handle
empty blocks correctly)

* Added _vec.cat_options, which takes a list of option[T] to a list
of T, ignoring any Nones

* Added two test cases.

working (on hello world at least):

~/inst/gdb/bin/gdb --args ./foo
(gdb) b write
...
(gdb) r
...
Breakpoint 1, 0xf7f04270 in write () from /lib32/libc.so.6
(gdb) bt
0  0xf7f04270 in write () from /lib32/libc.so.6
1  0x0804931a in rust_native_cdecl_3 ()
2  0x080487d7 in _rust_wrapper3_ ()
3  0x0804890a in _rust_fn5_main ()
4  0x08049440 in rust_native_cdecl_7 ()

rustc: Ignore the return value of native functions that return nil. stage1 can build libstd now, though it leaks.