collab: Add a Cents type (#18935)

This PR adds a new `Cents` type that can be used to represent a monetary
value in cents.

This cuts down on the primitive obsession we were using when dealing
with money in the billing code.

Release Notes:

- N/A

crates/collab/src/cents.rs

@@ -0,0 +1,78 @@
+/// A number of cents.
+#[derive(
+    Debug,
+    PartialEq,
+    Eq,
+    PartialOrd,
+    Ord,
+    Hash,
+    Clone,
+    Copy,
+    derive_more::Add,
+    derive_more::AddAssign,
+)]
+pub struct Cents(pub u32);
+
+impl Cents {
+    pub const ZERO: Self = Self(0);
+
+    pub const fn new(cents: u32) -> Self {
+        Self(cents)
+    }
+
+    pub const fn from_dollars(dollars: u32) -> Self {
+        Self(dollars * 100)
+    }
+
+    pub fn saturating_sub(self, other: Cents) -> Self {
+        Self(self.0.saturating_sub(other.0))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+
+    use super::*;
+
+    #[test]
+    fn test_cents_new() {
+        assert_eq!(Cents::new(50), Cents(50));
+    }
+
+    #[test]
+    fn test_cents_from_dollars() {
+        assert_eq!(Cents::from_dollars(1), Cents(100));
+        assert_eq!(Cents::from_dollars(5), Cents(500));
+    }
+
+    #[test]
+    fn test_cents_zero() {
+        assert_eq!(Cents::ZERO, Cents(0));
+    }
+
+    #[test]
+    fn test_cents_add() {
+        assert_eq!(Cents(50) + Cents(30), Cents(80));
+    }
+
+    #[test]
+    fn test_cents_add_assign() {
+        let mut cents = Cents(50);
+        cents += Cents(30);
+        assert_eq!(cents, Cents(80));
+    }
+
+    #[test]
+    fn test_cents_saturating_sub() {
+        assert_eq!(Cents(50).saturating_sub(Cents(30)), Cents(20));
+        assert_eq!(Cents(30).saturating_sub(Cents(50)), Cents(0));
+    }
+
+    #[test]
+    fn test_cents_ordering() {
+        assert!(Cents(50) > Cents(30));
+        assert!(Cents(30) < Cents(50));
+        assert_eq!(Cents(50), Cents(50));
+    }
+}