Allows to typecheck KernelDefinition

More uniform processing, at the cost of the encoding of 'restrict'
and 'extend' and not processing K_props and identity_props (they use
S_pred things).

Moved
- specs/tools/prelude to prelude,
- prelude patches to prelude/patches/
- the "simple" theory to tests/thy_translation/"
- scripts are moved to scripts/

- renamed subtype to subtype_poly
- pi function does not have implicit arguments anymore
- changed a rewrite rule into an axiom
- added wip tuples