Issue #25318: Add _PyBytesWriter API

Add a new private API to optimize Unicode encoders. It uses a small buffer
allocated on the stack and supports overallocation.

Use _PyBytesWriter API for UCS1 (ASCII and Latin1) and UTF-8 encoders. Enable
overallocation for the UTF-8 encoder with error handlers.

unicode_encode_ucs1(): initialize collend to collstart+1 to not check the
current character twice, we already know that it is not ASCII.