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/txhashset.rs

@@ -930,7 +930,7 @@ impl<'a> Extension<'a> {
 			if pmmr::is_leaf(n) {
 				if let Some(out) = self.output_pmmr.get_data(n) {
 					if let Some(rp) = self.rproof_pmmr.get_data(n) {
-						out.to_output(rp).verify_proof()?;
+						out.into_output(rp).verify_proof()?;
 					} else {
 						// TODO - rangeproof not found
 						return Err(Error::OutputNotFound);