Use an associated constant for type name in Describe

[randomPoison]

Blocked by rust-lang/rust#47032.

As of #15 the Describe trait has an associated function type_name to get the TypeName for a describable type. We'd prefer to use an associated constant, since a type's name should be fully known at compile time. However, to do that we need to change the type_params field of TypeName to be a Cow<'static, [TypeName]>, but there's currently a rustc bug rust-lang/rust#47032 that causes that code to not compile. Once rust-lang/rust#47032 has been fixed, we should switch to using an associated constant, and either deprecate or fully remove the type_name function.