Identity monad
Transformers proof-of-concept

By decomplecting the monadic value types from the monad's behaviour
having transformers is now possible. The existing API still works, but
there are still a few problems I'd like to address:

 * using monad transformers with mlet syntax;
 * automatic lifting within mlet in a transformer context (should be
   possible with `inner` and `lift`);
 * renaming of `core.lift` to `core.lift-m`;
 * adding back the set, list and lazy seq monads.
.clj -> .cljx
Either monad transformer
Rename monadic lift to lift-m and implement core.lift
Add ns declarations
Reorder continuation monad ns requires
Prepare core.cljx for working with monad transformers and fix tests
Move monad tests to their own file
cats.builtins and docs update
Add changelog
Separate builtin and core tests
Update docstring
Reimplement sequence monad
Reimplement and test set monad
Add license header to every source file
A few tests for the maybe transformer
Context protocol
Test lift in the maybe transformer
Docstring