I tried to pay attention to what was actually being tested so, e.g. when I
test was just using a vec as a boxed thing, I converted to boxed ints, etc.
Haven't converted the macro tests yet. Not sure what to do there.
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.
