store pmmr cleanup (#2075)

* introduce DataFile<T> to hide the LEN handling
use slices rather than vec<u8> where possible

* cleanup

* use DataFile<Hash> and not HashFile

* fix core tests

core/src/core/pmmr/backend.rs

@@ -59,7 +59,7 @@ pub trait Backend<T: PMMRable> {
 	/// Returns the data file path.. this is a bit of a hack now that doesn't
 	/// sit well with the design, but TxKernels have to be summed and the
 	/// fastest way to to be able to allow direct access to the file
-	fn get_data_file_path(&self) -> String;
+	fn get_data_file_path(&self) -> &str;
 
 	/// Also a bit of a hack...
 	/// Saves a snapshot of the rewound utxo file with the block hash as

core/src/core/pmmr/pmmr.rs

@@ -369,7 +369,7 @@ where
 	}
 
 	/// Return the path of the data file (needed to sum kernels efficiently)
-	pub fn data_file_path(&self) -> String {
+	pub fn data_file_path(&self) -> &str {
 		self.backend.get_data_file_path()
 	}
 

core/src/ser.rs

@@ -291,7 +291,7 @@ pub fn serialize<W: Writeable>(sink: &mut Write, thing: &W) -> Result<(), Error>
 /// Utility function to serialize a writeable directly in memory using a
 /// Vec<u8>.
 pub fn ser_vec<W: Writeable>(thing: &W) -> Result<Vec<u8>, Error> {
-	let mut vec = Vec::new();
+	let mut vec = vec![];
 	serialize(&mut vec, thing)?;
 	Ok(vec)
 }