Switch over to lua-resty-core and retire old CFunction-based Lua API impl

As originally discussed in #942, I'd like to discuss the option of forcibly switching over to lua-resty-core in this ngx_http_lua module and drop the existing CFunction-based Lua API implementation in the current ngx_http_lua module core.

The upside of this change is that we no longer have to maintain two parallel implementations of the same Lua API in both ngx_http_lua_module and lua-resty-core. And the ngx_http_lua_module core will be much smaller in terms of code size.

I propose the new lua_use_resty_core configuration directive for fine controls. By default, the resty.core module is automatically loaded right before running the init_by_lua* hook (if any). The user can avoid that by configuring the following directive in her nginx.conf:

lua_use_resty_core none;

Furthermore, the user can choose to load a selectively subset of submodules of resty.core.* like below:

# only load modules resty.core.regex, resty.core.shdict, and resty.core.req
lua_use_resty_core regex shdict req;

This leads to even a little bit smaller runtime memory footprint and less GC overhead (across those API function objects and containing Lua tables).

Or just load everything:

lua_use_resty_core all;

And loading everything under the resty.core.* namespace is the default.

A natural consequence of this change is that we also drop the long-time support of the standard Lua 5.1 interpreter in ngx_http_lua and the whole OpenResty. LuaJIT is already diverged a lot from it and it's becoming a big burden for us due to the lack of FFI in the standard Lua 5.1 interpreter, for example.

Another natural consequence of this is that ngx_http_lua_module will also depend on lua-resty-lrucache which is used by lua-resty-core's resty.core.regex submodule (for the compiled regex cache used by the ngx.re.* API).

Review #942 and the current ngx_http_lua module core alongside lua-resty-core, including the init_by_lua* hook and the proposed lua_use_resty_core directive in nginx.conf. Done means the old CFunction-based API is retired, selective or full resty.core loading works, standard Lua 5.1 support is removed, and the lua-resty-lrucache dependency is accounted for.