Optimize JSON string encoding

There are a couple of optimizations that work together: - We now use the specialized php_next_utf8_char_mb() helper function to avoid pressure on the µop and instruction cache. - It no longer emits UTF-8 bytes under PHP_JSON_UNESCAPED_UNICODE until it actually has to. By emitting in bulk, this improves performance. - Code layout tweaks * Use a specialized php_json_append() and assertions to avoid allocating the initial buffer, as this is already done upfront. * Factor out the call to smart_str_extend() to above the UTF-16 check to avoid code bloat. - Use SIMD, either with SSE2 or SSE4.2. A resolver is used when SSE4.2 is not configured at compile time.

Author: nielsdos

UPGRADING

@@ -457,6 +457,9 @@ PHP 8.5 UPGRADE NOTES
 14. Performance Improvements
 ========================================
 
+- JSON:
+ . Encoding JSON strings without special characters is now faster.
+
 - ReflectionProperty:
  . Improved performance of the following methods: getValue(), getRawValue(),
  isInitialized(), setValue(), setRawValue().