Optimize php_unicode_convert_case (cuts mbstring case conversion time ~15%)

Optimize php_unicode_convert_case (cuts mbstring case conversion time ~15%)

This function uses various subfunctions to convert case of Unicode wchars.
Previously, these subfunctions would store the case-converted characters in
a buffer, and the parent function would then pass them (byte by byte) to
the next filter in the filter chain.

Rather than passing around that buffer, it's better for the subfunctions to
directly pass the case-converted bytes to the next filter in the filter chain.
This speeds things up nicely.