core: Make Segment pub and serde (#574)

Turns out to be a useful type to send over the wire.

crates/core/src/db/message_log.rs

@@ -155,14 +155,20 @@ impl Default for OpenOptions {
     }
 }
 
-#[derive(Clone, Copy, Debug)]
-struct Segment {
-    min_offset: u64,
-    size: u64,
+/// A segment of a [`MessageLog`].
+///
+/// Represents a physical file holding a subset of the log.
+#[derive(Clone, Copy, Debug, serde::Serialize, serde::Deserialize)]
+pub struct Segment {
+    /// Message offset this segment starts with (zero-based).
+    pub min_offset: u64,
+    /// Size in bytes of this segment.
+    pub size: u64,
 }
 
 impl Segment {
-    fn name(&self) -> String {
+    /// Name of this segment, as it appears in its file name.
+    pub fn name(&self) -> String {
         format!("{:0>20}", self.min_offset)
     }
 }
@@ -480,6 +486,12 @@ impl SegmentView {
     }
 }
 
+impl From<SegmentView> for Segment {
+    fn from(SegmentView { info, .. }: SegmentView) -> Self {
+        info
+    }
+}
+
 impl TryFrom<SegmentView> for IterSegment {
     type Error = io::Error;