Introduce `ReservableCurrency` Trait in Balances Module (#2124)

* Introduce `ReservableCurrency`

* Update Docs for `ReservableCurrency`

* Update Tests

* Bump spec and rebuild wasm

* Extra: Add a note to `slash()`

Discussion in Riot clarified the behavior of `slash()` and `can_slash()`. Trying to sneak clarifying comments about it into this PR

* Update lib.rs

* Don't drop the periods!

CC @shawntabrizi