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.
floats. However, if someone writes a literal that can't be represented
precisely in 64 bits, the front-end will accept it while the back-end
will (presumably) complain.
