and rust_exit_task_glue calls the rust main.

This is simpler since we only need to setup one frame. It also matches
what ld.so does, so gdb is happy and stops a backtrace at rust_exit_task_glue
instead of continuing past whatever function happened to be before
rust_exit_task_glue is the object file.

This is only the rustc changes and should be merged first.

This ensures we don't get compile errors on unreachable code (see
test/run-pass/artificial-block.rs for an example of sane code that
wasn't compiling). In the future, we might want to warn about
non-trivial code appearing in an unreachable context, and/or avoid
generating unreachable code altogether (though I'm sure LLVM will weed
it out as well).

There was some confusion on whether the destructors took their
argument by pointer or direct value. They now take it directly, just
like other methods. You no longer get a segfault when a constructor
actually does something with its self value.

rustc: Refactor metadata.Encode.* to not require a type abbreviation table if abbreviation isn't enabled

This reverts commit d08b443f.

This reverts commit aa25f22f. It broke stage2, not sure why yet.

This giant commit changes the syntax of Rust to use "assert" for
"check" expressions that didn't mean anything to the typestate
system, and continue using "check" for checks that are used as
part of typestate checking.

Most of the changes are just replacing "check" with "assert" in test
cases and rustc.

Unlike rustboot, rustc keeps it destructors in vtables. Entry 0 holds
either the destructor for the obj or a NULL pointer. The method
offsets start at 1.

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.

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