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/module/src/symbol.rs

@@ -51,6 +51,7 @@ pub const DERIVABLE_ABILITIES: &[(Symbol, &[Symbol])] = &[
     (Symbol::ENCODE_ENCODING, &[Symbol::ENCODE_TO_ENCODER]),
     (Symbol::DECODE_DECODING, &[Symbol::DECODE_DECODER]),
     (Symbol::HASH_HASH_ABILITY, &[Symbol::HASH_HASH]),
+    (Symbol::EQ_EQ, &[Symbol::EQ_IS_EQ]),
 ];
 
 /// In Debug builds only, Symbol has a name() method that lets