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

substrate/node/runtime/src/lib.rs

@@ -58,7 +58,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("node"),
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
-	spec_version: 44,
+	spec_version: 45,
 	impl_version: 45,
 	apis: RUNTIME_API_VERSIONS,
 };