Add object attribute mechanism

This _kindof_ sucks.

This is my first idea of how to implement object attribute getters for
the object type.

Each type must provide an object-name-offset-type table, which gets
stored in the object v-table. The function `ws_object_attr_read()` allows
the _reading_ of such an attribute.

This code is untested by now and I consider this code as really ugly,
but I don't know a better way to do it by now. Please stay calm when
reviewing this.

Targets #64 
Would prefer `__ws_nonnull__` for self and dest.