Speed up rarity check while indexing (#702)

Replace check for uncommon or better rarity by specialized, more efficient
call. This method call contributed for 70% of the time in index_transaction, so
it's worth adding a more specialized method.

There's more potential here, as self.epoch() can likely be sped up
significantly - the binary search still takes an inordinate amount of time. See
issue #711.

src/index.rs

@@ -525,7 +525,7 @@ impl Index {
           .pop_front()
           .ok_or_else(|| anyhow!("insufficient inputs for transaction outputs"))?;
 
-        if Ordinal(range.0).rarity() > Rarity::Common {
+        if !Ordinal(range.0).is_common() {
           ordinal_to_satpoint.insert(
             &range.0,
             &encode_satpoint(SatPoint {

src/ordinal.rs

@@ -52,6 +52,14 @@ impl Ordinal {
     self.into()
   }
 
+  /// `Ordinal::rarity` is expensive and is called frequently when indexing.
+  /// Ordinal::is_common only checks if self is `Rarity::Common` but is
+  /// much faster.
+  pub(crate) fn is_common(self) -> bool {
+    let epoch = self.epoch();
+    (self.0 - epoch.starting_ordinal().0) % epoch.subsidy() != 0
+  }
+
   pub(crate) fn name(self) -> String {
     let mut x = Self::SUPPLY - self.0;
     let mut name = String::new();
@@ -597,4 +605,23 @@ mod tests {
       case(Ordinal::LAST.n() / (n + 1));
     }
   }
+
+  #[test]
+  fn is_common() {
+    fn case(n: u64) {
+      assert_eq!(
+        Ordinal(n).is_common(),
+        Ordinal(n).rarity() == Rarity::Common
+      );
+    }
+
+    case(0);
+    case(1);
+    case(50 * COIN_VALUE - 1);
+    case(50 * COIN_VALUE);
+    case(50 * COIN_VALUE + 1);
+    case(2067187500000000 - 1);
+    case(2067187500000000);
+    case(2067187500000000 + 1);
+  }
 }