Implement eris.sizeof() function from the LJ FFI module
Implement eris.sizeof() function from the LJ FFI module

This implements the eris.sizeof() module-level function, which behaves the
same way as the ffi.sizeof() function from the LuaJIT FFI module.

aperezdc/eris,aperezdc/eris,aperezdc/lua-eol,aperezdc/lua-eol