NLua for unity.

Support il2cpp.(use link.xml to avoid being clipped.)

copy example/Assets/UnityLua and example/Assets/Plugins to you project

The binaries in Plugings can be built from source. The solution files are in lua. For example, lua.dll can be built using /lua/proj.win

1. Remove KopiLua from NLua.
2. Make KeraLua support unity ios.
3. Remove Emit, because unity ios not support.
4. Remove some luanet_* functions. such as luanet_loadbuffer...
5. String use custom encoding, such as UTF8.
6. Disable search extension method because of the il2cpp.

1. Continue to support unity.