Fix missing Semigroup in ghc 8.0
Fix missing Semigroup in ghc 8.0

Prelude in ghc 8.0 does not come with Semigroup. Importing it directly
works in 8.0, 8.2, and 8.4.

reflex-frp/reflex,Saulzar/reflex,Saulzar/reflex,reflex-frp/reflex