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?

core/src/consensus.rs

@@ -221,7 +221,7 @@ where
 	let diff_sum = diff_data
 		.iter()
 		.skip(MEDIAN_TIME_WINDOW as usize)
-		.fold(0, |sum, d| sum + d.clone().unwrap().1.into_num());
+		.fold(0, |sum, d| sum + d.clone().unwrap().1.to_num());
 
 	// Apply dampening except when difficulty is near 1
 	let ts_damp = if diff_sum < DAMP_FACTOR * DIFFICULTY_ADJUST_WINDOW {