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.
It was not obvious how to make this implementation work when the unit size
was not also a power of two, so for now just make the buffer size a multiple
of the unit size so it can pass all the tests.
