Syntax is of the form

    pattern P :: (Prov b) => (Req a) => a -> b -> Int -> T a

which declares a pattern synonym called `P`, with argument types `a`, `b`,
and `Int`, and result type `T a`, with provided context `(Prov b)` and required
context `(Req a)`.

The Haddock submodule is also updated to use this new syntax in generated docs.