Parametrise modules

I would like to be able to parametrise modules by types and lifetimes. Type parameters are useful where many items in a module should use the same concrete type. E.g., taking some implementation as a parameter we want to ensure that all functions and data types in a module use the same implementor without annotating every item with the same type parameters. Likewise, parameterising by lifetimes is useful if we are to assume that many objects in a module have the same lifetime. This is especially useful in conjunction with arena allocation.

Details

Module declarations may have formal type and lifetime parameters and where clauses, e.g, mod foo<X, 'a> where X: Bar { ... }.

Module uses, including in use expressions which include an alias, can have actual type and lifetime parameters. E.g, let x: foo<int>::Baz = ...; or use foo<int, 'static> as int_foo.

The usual rules around well-formedness wrt bounds, and inference would apply.

Start by reviewing the proposed module declarations and module-use examples in this issue, then compare them with Rust's existing module, type-parameter, lifetime, and where-clause rules. Done means an agreed design or RFC that specifies bounds, inference, and use-alias behavior; no repository file or test is named here.