Replace aliases in Aliases_of_canonical_element.t with 3 fields

`Aliases_of_canonical_element.t` carries around a map from `Name_mode.t`
to `Simple.Set.t` so that it can compute the earliest element in each
name mode. This is less efficient than simply having a field for each
name mode. Furthermore, we don't need the sets if we just track the
earliest for each name mode (along with its binding time).

Author: lukemaurer