codecvt: Re-implement do_out and do_unshift

The previous implementation assumed that only one byte would be given
in the `from` buffer by the caller at a time.  This may be true for
MSVC but is not for the GNU library on Windows.  Re-implement these
methods to handle more than one byte per call.

Also simplify the state management by keeping all state between calls
directly in the `mbstate_t` argument instead of using it to index our
own heap-allocated state.

Fixes: #16893

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