feature: Re-implement ngx_lua's ngx.decode_args API function with FFI

[hongliang5316]

One of our projects had used a lot of ngx.decode_args API function.
When i make a performance test for this project on the Flame Graph. I found the ngx.decode_args API function occupied most of CPU utilization, which reduce QPS seriously. So i re-implement the ngx.decode_args API function with FFI.
I use this code to test:

--10 args
 local args = {
     a = "a", b = true, c = "c", d = "d",
     e = "e", f = "f", g = "g", h = "h",
     i = "i", j = "j"
 }

 local str = ngx.encode_args(args)

 local now = ngx.time()

 for i = 1, 1e6 do
     local _ = ngx.decode_args(str)
 end

 ngx.update_time()
 ngx.say(ngx.now() - now)

The result is:
for lua c api
2.7109999656677
for ffi
1.3810000419617

It seems 100% faster, that's great!
@agentzh
What do you think about this PR?
And also openresty/lua-nginx-module#1108.
Thanks!

[spacewander]

It seems FFI+JIT is a gold mine. 😃

[hongliang5316]

+1. 😄

[agentzh]

@hongliang5316 This is awesome! I'll have a look as soon as I can manage.

BTW, you can often get better performance in a whole OpenResty application by adding the following line to your init_by_lua* handler:

require "jit.opt".start("minstitch=3")