metamethods for containers represented using userdata,
`#` == __len
`[key] = value` == __newindex
`key` == __index
are not working properly for some reason, will need to investigate more deeply to find out why
on bright side all tests pass including new tests added (for take/return std::function)
fixed some things forgot to change with addition of upvalue_t for clarity (see previous commit about userdata/lightuserdata vs upvalue)