Implement obligation checking for the Eq ability

Every type can have `Eq.isEq` derived for it, as long as

- it does not transitively contain a function
- it does not transitively contain a floating point value
- it does not transitively contain an opaque type that does not support
  `Eq`

crates/compiler/solve_problem/src/lib.rs

@@ -67,9 +67,15 @@ pub enum NotDerivableContext {
     UnboundVar,
     Opaque(Symbol),
     Decode(NotDerivableDecode),
+    Eq(NotDerivableEq),
 }
 
 #[derive(PartialEq, Debug, Clone)]
 pub enum NotDerivableDecode {
     OptionalRecordField(Lowercase),
 }
+
+#[derive(PartialEq, Debug, Clone)]
+pub enum NotDerivableEq {
+    FloatingPoint,
+}