This allows stateful or configurable codecs without having to express
all configuration in the type name itself. For example, we could have a
Base64 type with configurable Base64 engines without having to duplicate
the Base64 type itself.
(Note that the different engines in the Base64 crate are values, not
types.)
In 1265f4b, we introduced a change which may cause a conflict of type
names when deriving the traits on two different types. While a
workaround existed (use `mod`s to isolate the implementation), that is
ugly.
This commit allows overriding the choice of type names.
