remove the cached comparison in literals
added Signal to the lib/
small change to the API of Literal
started refactoring so that Env, Ctx, Clause are module with global state
but created by a functor so that several instances are possible
updated lib/Signal so it uses a more descriptive variant
updated interfaces files for the Great Functorization, including
Env and Extensions (now a first-class functor)
refactored most modules so that they are functors
over Env/Ctx/Clause, and use this to make Superposition a mere extension
bugfix
Change the internal system so that Env and Ctx use global constants
encapsulated in functors (so they can be duplicated).

Merge from state-in-functors
more oriented equations
ready for the current version of Logtk

use the new API for subsumption index