Eliminate harmless non-constant time operations on secret data.

There were several places where the code was non-constant time
 for invalid secret inputs.  These are harmless under sane use
 but get in the way of automatic const-time validation.

(Nonce overflow in signing is not addressed, nor is s==0 in
 signing)

src/field_impl.h

@@ -315,4 +315,6 @@ static int secp256k1_fe_is_quad_var(const secp256k1_fe *a) {
 #endif
 }
 
+static const secp256k1_fe secp256k1_fe_one = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 1);
+
 #endif /* SECP256K1_FIELD_IMPL_H */