PMMR data compaction (#784)

* fix and re-introduce data file compaction

* rustfmt

* remove commented line

* test cleanup

* remove println

* fix to core test

core/src/core/pmmr.rs

@@ -63,14 +63,18 @@ where
 	/// occurred (see remove).
 	fn rewind(&mut self, position: u64, index: u32) -> Result<(), String>;
 
-	/// Get a Hash/Element by insertion position. If include_data is true, will
+	/// Get a Hash by insertion position. If include_data is true, will
 	/// also return the associated data element
 	fn get(&self, position: u64, include_data: bool) -> Option<(Hash, Option<T>)>;
 
-	/// Get a Hash/Element by original insertion position (ignoring the remove
+	/// Get a Hash  by original insertion position (ignoring the remove
 	/// list).
 	fn get_from_file(&self, position: u64) -> Option<Hash>;
 
+	/// Get a Data Element by original insertion position (ignoring the remove
+	/// list).
+	fn get_data_from_file(&self, position: u64) -> Option<T>;
+
 	/// Remove HashSums by insertion position. An index is also provided so the
 	/// underlying backend can implement some rollback of positions up to a
 	/// given index (practically the index is the height of a block that
@@ -1027,6 +1031,14 @@ mod test {
 			}
 		}
 
+		fn get_data_from_file(&self, position: u64) -> Option<T> {
+			if let Some(ref x) = self.elems[(position - 1) as usize] {
+				x.1.clone()
+			} else {
+				None
+			}
+		}
+
 		fn remove(&mut self, positions: Vec<u64>, _index: u32) -> Result<(), String> {
 			for n in positions {
 				self.remove_list.push(n)