Allow pattern-matching in lets (and other definitions) for records and other single-constructor types

[lukaszcz]

We should allow:

let (x, y) := f a
in

and

let record@{field1; field2} := f a
in

This is convenient syntactic sugar which easily translates to pattern matching. We should allow it only for single-constructor types so there is no issue with non-exhaustive patterns.

For consistency, we can allow it everywhere where a function definition is expected.