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