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?

chain/src/chain.rs

@@ -30,8 +30,8 @@ use pipe;
 use store;
 use txhashset;
 use types::*;
-use util::secp::pedersen::{Commitment, RangeProof};
 use util::LOGGER;
+use util::secp::pedersen::{Commitment, RangeProof};
 
 /// Orphan pool size is limited by MAX_ORPHAN_SIZE
 pub const MAX_ORPHAN_SIZE: usize = 200;
@@ -260,7 +260,7 @@ impl Chain {
 		debug!(
 			LOGGER,
 			"Chain init: {} @ {} [{}]",
-			head.total_difficulty.into_num(),
+			head.total_difficulty.to_num(),
 			head.height,
 			head.last_block_h,
 		);