Rework reading from device and clean up.

src/mapping/domain.rs

@@ -0,0 +1,22 @@
+use serde::{Deserialize, Serialize};
+
+/// Domain, in sectors.
+/// Requires sector_size to be provided elsewhere for conversion to bytes.
+#[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Domain {
+    pub start: usize,
+    pub end: usize,
+}
+
+impl Default for Domain {
+    fn default() -> Self {
+        Domain { start: 0, end: 1 }
+    }
+}
+
+impl Domain {
+    /// Return length of domain in sectors.
+    pub fn len(self) -> usize {
+        self.end - self.start
+    }
+}