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?

servers/src/common/stats.rs

@@ -15,8 +15,8 @@
 //! Server stat collection types, to be used by tests, logging or GUI/TUI
 //! to collect information about server status
 
-use std::sync::{Arc, RwLock};
 use std::sync::atomic::AtomicBool;
+use std::sync::{Arc, RwLock};
 use std::time::SystemTime;
 
 use chain;
@@ -165,7 +165,7 @@ impl PeerStats {
 			state: state.to_string(),
 			addr: addr,
 			version: peer.info.version,
-			total_difficulty: peer.info.total_difficulty.into_num(),
+			total_difficulty: peer.info.total_difficulty.to_num(),
 			height: peer.info.height,
 			direction: direction.to_string(),
 		}