Add `instance Semigroup a => Semigroup (IO a)`

What the title says

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett
Operating system
Architecture

changed weight to 5

added Tfeature request Trac import core libraries labels

Trac metadata

Trac field	Value
Differential revisions	- → D2588

mentioned in commit 8c6a3d68

closed

changed milestone to %8.2.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Pnormal label