std: move io module out of pal, get rid of sys_common::io

[joboet]

Part of #117276.

This does two related things:

1. It moves the platform-specific definitions for IoSlice, IoSliceMut and is_terminal out of pal and into sys and unifies some of them.
2. It gets rid of sys_common::io, moving the non-platform-specific test helpers into std::test_helpers and the buffer size definition to the new sys::io module.