Avoid class special handling of _class, _type and other elements

_element method is used for both queying class attributes and instance
attributes.  However, _class, _type etc. have special meaning only for
instance access, so turn off its special handling when _element(object=nil)

Also robustify core.record.new() - it should not let anything other
than table as the type, otherwise bug in lua code can crash the whole
Lua VM.

This fixes https://github.com/pavouk/lgi/issues/40