libscript-lua: use integer APIs of Lua 5.3 for script_get_int and script_put_int #2

Lua 5.3 supports 64-bit integers. When building libscript-lua with Lua 5.3, script_get_int and script_put_int should send and receive 64-bit integers without losing precision. The library should continue to work with Lua 5.1/5.2 (if necessary, the code should be marked #if LUA_VERSION_NUM >= 503 so that it is used only in Lua 5.3).

A way to test that 64-bit integer support is working is that this number (2^53+3) should be sent to the script and retrieved without precision loss: 9007199254740995 (if this number is accidentally converted to a 64-bit float somewhere down the line it will lose one bit of precision and render as 9007199254740996).