Move trans, back, driver, and back into a new crate, rustc_trans. Reduces memory usage significantly and opens opportunities for more parallel compilation.
auto merge of #19070 : nikomatsakis/rust/crates, r=nikomatsakis

Reduces memory usage significantly and opens opportunities for more parallel compilation.

This PR was previously #19002 but I closed it because bors didn't seem to recognize the `r+` annotations there.
Make os::getcwd() return IoResult<Path>

os::getcwd() panics if the current directory is not available. According
to getcwd(3), there are three cases:

- EACCES: Permission denied.
- ENOENT: The current working directory has been removed.
- ERANGE: The buffer size is less than the actual absolute path.

This commit makes os::getcwd() return IoResult<Path>, not just Path,
preventing it from panicking.

As os::make_absolute() depends on os::getcwd(), it is also modified to
return IoResult<Path>.

Fixes #16946.

[breaking-change]
Make os::change_dir() return IoResult<()>

os::change_dir() returns bool, without a meaningful error message.
Change it to return IoResult<()> to indicate what IoError caused the
failure.

Fixes #16315.

[breaking-change]
Make os::setenv() and os::unsetenv() panic if an error occurs

These functions can fail if:

- EINVAL: The name is empty, or contains an '=' character
- ENOMEM: Insufficient memory