Allow `TypeParam`s in `TypeArg`s.

[zrho]

TypeParam encodes static types while TypeArg encodes the values of static types. A static type itself is statically known information and so merits to be passed as an argument. This enables expressing the types of various constructions declaratively, improves consistency and shrinks the gap with hugr-model. In particular:

• TypeArg should be able to embed TypeParam.
• TypeParam should gain a variant TypeParam::Static that is the type of static types.
• TypeParam should allow variables.

[zrho]

Instead of allowing to embed TypeParams in TypeArgs, we may also merge TypeParam and TypeArg. This makes future work clearer.