…dedInterfaces

Replace hand-rolled pointer/interface unwrap loops with deref.Value and
deref.Type, consistent with how Fetch already dereferences values.

Add white-box unit tests covering each branch (100% coverage of the
function): plain struct, embedded interface with/without field, pointer
to struct concrete value, nil concrete value, nil embedded pointer,
nested embedded struct, nested embedded interface, non-struct concrete
value, expr:"-" tag, and empty concrete struct.