Return base field element from commit_ivk()

Co-authored-by: Jack Grigg <jack@electriccoin.co>

orchard_commitments.py

@@ -44,11 +44,11 @@ def rcm_trapdoor(rand):
 
 # https://zips.z.cash/protocol/nu5.pdf#concreteorchardnotecommit
 def commit_ivk(rivk: Scalar, ak: Fp, nk: Fp):
-    return Scalar(sinsemilla_short_commit(
+    return sinsemilla_short_commit(
         rivk,
         b"z.cash: Orchard-CommitIvk",
         i2lebsp(L_ORCHARD_BASE, ak.s) + i2lebsp(L_ORCHARD_BASE, nk.s)
-    ).s)
+    )
 
 def rivk_trapdoor(rand):
     return Scalar.random(rand)

orchard_pallas.py

@@ -229,6 +229,7 @@ class Point(object):
         return self.x
 
     def __mul__(self, s):
+        assert isinstance(s, Scalar)
         s = format(s.s, '0256b')
         ret = self.ZERO
         for c in s: