Replaced LuaCSFunction with lua.CFunction.
Added lua.pushcfunction/pushcclosure/register/cpcall.
Changed all code to use lua.pushcfunction instead of luanet.pushstdcallcfunction and set lua.CFunction to marshal as cdecl.
(I have no idea why they didn't do this from the beginning. Maybe they didn't know you could set the calling convention. A test is included which shows that this change nearly doubles function call performance.)
Fixed the lua.Reader calling convention.
Removed LuaFunctionCallback. lua.atpanic now takes a standard lua.CFunction. (It used to use the wrong calling convention.)