Rollup merge of #140393 - joboet:sys_common_process, r=thomcc

std: get rid of `sys_common::process`

Move the public `CommandEnvs` into the `process` module (and make it a wrapper type for an internal iterator type) and everything else into `sys::process` as per #117276.

Something went wrong with a force push, so I can't reopen #139020. This is unchanged from that PR, apart from a rebase.

r? ```@thomcc```

Author: Zalathar