Prior to this change, alias types would be added to the `processor.Types`
map twice, in the case where an API defines both the alias type, and
another struct which uses either a pointer to, or an array / slice of,
an alias type. Depending on which element is processed first, the alias
type would be added to the map as an AliasKind, or a
SliceKind/ArrayKind/PointerKind. This means that where templates attempt
to render an underlying type of an Alias using `IsAlias`, the alias may
be registered as a non-alias type.

In addition, alias types were treated as Basic in the renderer if their
underlying types were basic. This means in the above case, links are not
generated to the alias type in the generated documentation, despite them
being a valid type as part of the API.