data Unit for 2.4.0 compatibility

Agda-2.4.0 does not allow COMPILED_DATA for records, so
we need a special Unit datatype.
Merge branch 'master' into maint-2.4.0
Merge branch 'master' into maint-2.4.0
Merge branch 'master' into maint-2.4.0
Build test/Main.agda with agda-2.4.0
Merge branch 'master' into maint-2.4.0
Merge branch 'master' into maint-2.4.0

Conflicts:
	src/Prelude/Unit.agda
Merge branch 'master' into maint-2.4.0
Merge branch 'master' into maint-2.4.0
Fixed typo in definition of if' in Prelude.Bool