As a side effect, this fixes a bug where typestate would not properly be checked
inside of fn expressions.

Modify the fn vistors in walk so that they can handle functions without names. Update the typestate code to understand this.

rustc: Increment by the unit size in iter_structural_ty when iterating over interior vectors if the unit size is dynamic

Typeck and trans used to, by historical coincidence, use the item_obj
node id, which was used to identify the obj type by the rest of the
system, for the constructor function. This is now identified by the
ctor id stored in the tag throughout.

This in preparation of making 'res' a keyword for defining resources.
Please don't introduce too many new ones in the meantime...

Since obj constructors and types have different def_ids now,
their def can simply be a def_fn and a def_ty.

The implementation is so simple it might be considered cheating: at almost
every step the expr_ternary is just converted to expr_if.

Wrote swap_context in assembly. Closes #548. This also allows us to re-enable optimization on OSX for both gcc and clang. Also, re-enamed task-comm-1.rs, which was previously broken by optimization.

We decided to use metadata for the more complex cases, and a simple
string is enough for rustc right now.

If you use a function expecting an alias argument in a context that
expects a function expecting a value argument, or vice versa, the
previous error message complained that the number of arguments was
wrong. Fixed the error message to be accurate.

typestate now drops constraints correctly in the post-state of
a move expression or a declaration whose op is a move. It doesn't
yet drop constraints mentioning variables that get updated.

To do this, I had to change typestate to use trit-vectors instead
of bit-vectors, because for every constraint, there are three
possible values: known-to-be-false (e.g. after x <- y, init(y) is
known-to-be-false), known-to-be-true, and unknown. Before, we
conflated known-to-be-false with unknown. But move requires them
to be treated differently. Consider:

(program a)
(a1) x = 1;
(a2) y <- x;
(a3) log x;

(program b)
(b1) x = 1;
(b2) y <- z;
(b3) log x;

With only two values, the postcondition of statement a2 for
constraint init(x) is the same as that of b2: 0. But in (a2)'s
postcondition, init(x) *must* be false, but in (b2)'s condition,
it's just whatever it was in the postcondition of the preceding statement.

Closes #367

This makes the error message correctly identify the term that wasn't the
expected type. Issue #516.

This is so that subsequent reports about type mismatches get the types
correct.

Issue #516

module, not just everything exported.

Examples: #[test], #[link(name = "vers")]

Issue #487