tock-registers: Derive Default, Eq and PartialEq for FieldValue.

Default is useful to construct a FieldValue which can be passed to
`modify` as a no-op or used as an additive identity when combining other
FieldValues.

Eq and PartialEq are useful for assertions in testing.

Author: qwandor

libraries/tock-register-interface/src/fields.rs

@@ -190,7 +190,7 @@ Field_impl_for!(usize);
 ///
 /// For the FieldValue, the masks and values are shifted into their actual
 /// location in the register.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Default, Eq, PartialEq)]
 pub struct FieldValue<T: UIntLike, R: RegisterLongName> {
     mask: T,
     pub value: T,