Rustify core/src/core (#1122)

Small refactoring of one folder, if it makes sense I could extend the scope.
* Remove some cloning (real and just verbosity in the code)
* Naming conventions like to/into*
* Some Clippy's suggestions

I found that we don't use field init shorthand syntax, so I didn't touch this part, was it discussed before?

api/src/handlers.rs

@@ -568,7 +568,7 @@ impl BlockHandler {
 			))?;
 		}
 		let vec = util::from_hex(input).unwrap();
-		Ok(Hash::from_vec(vec))
+		Ok(Hash::from_vec(&vec))
 	}
 }