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.
Add a Data.Maybe.Minimal module
[ Data.Unit ] Removed unnecessary importation.
Renamed Data.Unit.Core to Data.Unit.NonEta

Data.Unit.Core isn't a "Core" module (i.e. a module to avoid mutual
recursion between modules).
Merge branch 'unit' of /home/asr/agda-stdlib-2.4.2.1

Conflicts:
	src/Data/Unit.agda
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