Added Unit = ⊤ to Prelude.Unit to minimize diff to maint-2.4.0
Merge branch 'master' into maint-2.4.0

Conflicts:
	src/Prelude/Unit.agda
Main can have type IO ⊤ even if ⊤ is not a Haskell type.

The result of main is silently discarded.
Merge branch 'master' into maint-2.4.0