As per Graydon's comments on #1985: remove char::to_lower and
char::to_upper. The str versions of these functions now call
libc::tolower and libc::toupper (using wrappers that cast between
char and c_char). Not sure how much better that is, but it at least
makes it clearer that these functions are Unicode-unsafe.
- Move io, run and rand to core.
- Remove incorrect ctypes module (use libc).
- Remove os-specific modules for os and fs.
- Split fs between core::path and core::os.
