Implement try_mutate for storage value and storage double map (#5699)

* impl try_mutate for storage value and storage double map

* Docs + Reuse `try_mutate` in `mutate`

substrate/frame/support/src/lib.rs

@@ -78,6 +78,10 @@ pub use self::storage::{
 pub use self::dispatch::{Parameter, Callable, IsSubType};
 pub use sp_runtime::{self, ConsensusEngineId, print, traits::Printable};
 
+/// A type that cannot be instantiated.
+#[derive(Debug)]
+pub enum Never {}
+
 /// Macro for easily creating a new implementation of the `Get` trait. Use similarly to
 /// how you would declare a `const`:
 ///