Faster luaL_addlstring for LuaJIT and Lua 5.1

[Domingo Alvarez Duarte <mingodad@...>]

Hello !

Converting LuaJIT to LjsJIT https://github.com/mingodad/ljsjit and 
making some tests I found that the api function luaL_addlstring in lua 
5.1 and LuaJIT is a lot slower than more recent Lua versions so using 
the luaL_addvalue as base I reimplemented luaL_addlstring and it's now 
on par with lua 5.3.5 .

https://github.com/LuaJIT/LuaJIT/pull/465

Cheers !