Support type aliases

[flupe]

The typed lambox target supports compiling type schemes (i.e definitions whose type is an arity) into type aliases.

See section 5.1.1 (https://arxiv.org/pdf/2108.02995).

We have TypeAliasDecl in our environment AST, let's support those.

Note: there's no point in compiling logical type schemes, i.e type aliases that produce propositions.
So really, the type schemes that should be supported must have a type that is an arity into Type.