Add match expressions

[jmackie]

Also known as: pattern matching, case expressions.

Initial syntax should look like:

match expr with
| Foo(x) -> do_something_with(x)
| Bar -> y
| _ -> z

So for this first pass, we're only worried about constructor patterns and wildcard patterns.

Explicitly not in scope:

1. Literal patterns
2. Exhaustiveness checking
3. Arbitrarily nested OR patterns (see this rust RFC)

1 and 2 will be fast followers. 3 will come much later (if at all).

• Add match expressions #22
• ditto-lang/vim-ditto@b80652e
• ditto-lang/tree-sitter-ditto@411a3bf