Simplify the type of Concurrently (#10)

The instances now require UndecidableInstances, which is, I think,
not that bad.
Fix haddock comments
Merge branch 'feature/simpler-types' into develop
Bump to v0.5.0
Merge branch 'release/v0.5.0'
Merge tag 'v0.5.0' into develop

v0.5.0