Add a "with" syntax for patterns

Add a "with" syntax for patterns

• Expertise: ★★★★☆
• Time: ★★★★☆
• Mantis: None
• Mentor: Jeremy/Leo
• Who is working on this: ???
• What needs to be done: Add the following construct

match x with
  Foo(y, z)
| Bar y with z = 3 -> y + z

The with z = 3 binds z to 3 if the Bar clause in the or-pattern matches.

See the ocaml-patterns documentation for a more detailed semantics.