Introduce Data.Unit.Minimal

This module avoids importing from Relation.Binary.*
Rework Data.Bool.Core as Data.Bool.Minimal

Data.Bool.Minimal only imports "minimal" modules
(here Data.Unit.Minimal and Data.Empty).

Additionally to the type the simple operations on
boolean values have been moved to Data.Bool.Minimal.

These minimal modules helps cutting down the number of
modules to be compiled.