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.
Nicer parsing of self-calls (expr_self_method nodes inside expr_call
nodes, rather than a separate expr_call_self) makes typechecking
tractable. We can now write self-calls that take arguments and return
values (see: test/run-pass/obj-self-*.rs).