Implement minor performance improvements
Implement minor performance improvements

- Use strbuf_append_mem() for small static strings (~2% speedup).
- Use &json_config_key for storing registry data. It's more unique
  and faster than a text string.
- Use strbuf_append_char_unsafe() for string quotes (~4% speedup).
- Use strbuf_append_number() instead of strbuf_append_fmt(). It is
  much simpler and avoids the potential for 2 expensive calls to
  vsnprintf().
- Make encoding buffer persistent across calls to avoid extra
  malloc/free (~4% speedup on example2.json).

These performance improvements can be much more pronounced depending
on the data. Eg, small strings, numbers, booleans, etc..

msva/lua-cjson,hynd/lua-cjson,mah0x211/lua-cjson,msva/lua-cjson,brimworks/lua-cjson,hilllinux/lua-cjson,brimworks/lua-cjson,hilllinux/lua-cjson,brimworks/lua-cjson,goecho/lua-cjson,hynd/lua-cjson,openresty/lua-cjson,mpx/lua-cjson,goecho/lua-cjson,hnutank163/lua-cjson,ovx/lua-cjson,singlecomm/lua-cjson,mythay/lua-cjson,drmingdrmer/lua-cjson,msva/lua-cjson,cloudwu/lua-cjson,openresty/lua-cjson,openresty/lua-cjson,singlecomm/lua-cjson,goecho/lua-cjson,mpx/lua-cjson,ovx/lua-cjson,mah0x211/lua-cjson,hilllinux/lua-cjson,drmingdrmer/lua-cjson,mpx/lua-cjson,hynd/lua-cjson,mythay/lua-cjson,cloudwu/lua-cjson,drmingdrmer/lua-cjson,cloudwu/lua-cjson,hnutank163/lua-cjson,singlecomm/lua-cjson,mah0x211/lua-cjson,ovx/lua-cjson,mythay/lua-cjson,hnutank163/lua-cjson