Proposal: Amend or patterns (#522) to use p1 ; p2 (#609)

Author: Sebastian Graf, David Knothe

Rendered proposal: https://github.com/ghc-proposals/ghc-proposals/blob/eb4b67c29282520b2c5c6a49c3047dbecb15dde1/proposals/0522-or-patterns.rst

Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/609

Recommendation: Acceptance

This proposal refines the syntax of an already accpted proposal (#522) for writing or patterns.

The syntax of the old proposal,

```haskell

stringOfT :: T -> Maybe String

stringOfT (T1 s) = Just s

stringOfT (one of T2{}, T3{}) = Nothing

```

is replaced with

```haskell

stringOfT :: T -> Maybe String

stringOfT (T1 s) = Just s

stringOfT (T2{}; T3{}) = Nothing

```

or indeed

```haskell

stringOfT :: T -> Maybe String

stringOfT x = case x of

T1 s -> Just s

T2{}

T3{} -> Nothing

```

(Beacuse the '(' ... ')' are optional here, we can remove them and naturally exploit layout.)

The authors have conducted polls to see what folks preferred, landing on this proposal. The

adjustments to the Haskell grammar are really quite minimal requiring one extra production.

The proposal is nondiruptive and as a whole addresses an important practical topic. I recommend

that we accept it