Except for _task, which is still a keyword.

This should be a snapshot transition.

(Have fun mergining your stuff with this.)

This reverts commit d08b443f.

This reverts commit aa25f22f. It broke stage2, not sure why yet.

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.

Finally implements _str.is_utf8, adds from_chars, from_char, to_chars,
char_at, char_len, (push|pop|shift|unshift)_char. Also, proper
character I/O for streams.

Switch all vases of vec += elt to vec += vec. Prohibit former in rustboot. Tweak std lib vec fns in process.

std.fs.list_dir will list the files in a directory, std.fs.file_is_dir
will, given a pathname, determine whether it is a directory or not.