Initial commit, including:

- ListTheory.agda
-- Empty and singleton lists are identities for `reverse`
-- `reverse` is an involution

- SetTheory.agda
-- Extensional equivalence
--- Proof that it is an equivalence relation
-- Union (commutative) monoid
-- Intersection (commutative) monoid
Added Languages.agda, including:
- Definitions of strings and languages
- Concatenation
-- Monoid
- Kleene closure
-- Superset (singleton)
-- Left-append
-- Idempotency
- Reversal
-- Is an involution
- Concatenation is idempotent on the Kleene star
- Reversal commutes with concatenation
Merge branch 'master' of https://github.com/Nosewings/agda-formal-languages