Add ScalarEngine trait

This is extracted from pairing's Engine trait.

src/lib.rs

@@ -290,6 +290,14 @@ pub trait PrimeField: Field {
     fn root_of_unity() -> Self;
 }
 
+/// An "engine" is a collection of types (fields, elliptic curve groups, etc.)
+/// with well-defined relationships. Specific relationships (for example, a
+/// pairing-friendly curve) can be defined in a subtrait.
+pub trait ScalarEngine: Sized + 'static + Clone {
+    /// This is the scalar field of the engine's groups.
+    type Fr: PrimeField + SqrtField;
+}
+
 #[derive(Debug)]
 pub struct BitIterator<E> {
     t: E,