add identity proof.
change symbols.
add composition proof and change composable helper module.
add homomorphism proof.
add homomorphism for nothing case (even if it is kind of obvious)
demonstrate map equivalence.
add space for interchange.
remove comment.
fix homomorphism proof (oops!)
the value returned from f and g should be rand on a per-run basis.
remove variable assignment and use a lambda binding instead.
"prove" interchange law holds.
add todo.
Merge pull request #12 from unsymbol/feature/prove-maybe-is-an-applicative

Prove maybe is an applicative

closes https://github.com/unsymbol/ribimaybe/issues/10