Use different syntax for checks that matter to typestate

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.

src/lib/linux_os.rs

@@ -66,7 +66,7 @@ fn dylib_filename(str base) -> str {
 
 fn pipe() -> tup(int, int) {
     let vec[mutable int] fds = vec(mutable 0, 0);
-    check(os.libc.pipe(_vec.buf[mutable int](fds)) == 0);
+    assert (os.libc.pipe(_vec.buf[mutable int](fds)) == 0);
     ret tup(fds.(0), fds.(1));
 }
 
@@ -76,7 +76,7 @@ fn fd_FILE(int fd) -> libc.FILE {
 
 fn waitpid(int pid) -> int {
     let vec[mutable int] status = vec(mutable 0);
-    check(os.libc.waitpid(pid, _vec.buf[mutable int](status), 0) != -1);
+    assert (os.libc.waitpid(pid, _vec.buf[mutable int](status), 0) != -1);
     ret status.(0);
 }