Rollup merge of #119996 - joboet:move_pal_os_str, r=ChrisDenton

Move OS String implementation into `sys`

Part of #117276. The new structure is really useful here, since we can easily eliminate a number of ugly `#[path]`-based imports.

In the future, it might be good to move the WTF-8 implementation directly to the OS string implementation, I cannot see it being used anywhere else. That is a story for another PR, however.

Author: Nadrieril