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.
I think just about every type can be used as a block result now. There's quite
a proliferation of tests here, but they all test slightly different things and
some are split out to remain XFAILed. The tests of generic vectors are still
XFAILed because generic aliased boxes still don't work in general.
