Constrained type syntax and parsing

[maximemulder]

My first PR after one year in the group lmao.

Description

This PR adds syntax, parsing, and elaboration for constrained types, using the following syntax:

[(type (<: |: >) type)*] => type

Example:

[A <: Int, Str :> B] => Bool

Design choices

• Both subtyping (<:) and supertyping (:>) constraint syntaxes are accepted.
• A single constrained type can have an arbitrary number of constraints (0 to n).
• I added some parsing tests (Tree) but I did not see any dedicated place to add elaboration tests (Term).

[LPTK]

Otherwise LGTM.