Measure heap usage while processing the ledger

measure/src/thread_mem_usage.rs

@@ -28,6 +28,7 @@ impl Allocatedp {
         Self {}
     }
 
+    /// Return current thread heap usage
     pub fn get(&self) -> u64 {
         #[cfg(unix)]
         {
@@ -36,4 +37,9 @@ impl Allocatedp {
         #[cfg(not(unix))]
         0
     }
+
+    /// Return the difference in thread heap usage since a previous `get()`
+    pub fn since(&self, previous: u64) -> i64 {
+        self.get() as i64 - previous as i64
+    }
 }