ivy: fix bug in up and down (known as grade internally)

They were using binary <, which works elementwise. This is what
OrderedCompare is for.

One of the cases where allowing non-scalar elements required changing
existing code.

Fixes #189

testdata/binary_vector.ivy

@@ -498,6 +498,9 @@ rho (6 2 rho "abcdefghijkl") [3 2 rho iota 6]
 x = 1 rho 1; rho x[up x]
 	1
 
+up 'ab' 'ac' 'aa'
+	3 1 2
+
 3 sel 1
 	1 1 1
 

testdata/unary_vector.ivy

@@ -236,3 +236,6 @@ mix split 2 2 rho iota 4
 	1 2
 	3 4
 
+up mix 'ab' 'ac' 'aa'
+	3 1 2
+

value/matrix.go

@@ -966,10 +966,11 @@ func (m *Matrix) grade(c Context) *Vector {
 		i = x[i] * stride
 		j = x[j] * stride
 		for k := 0; k < stride; k++ {
-			if toBool(c.EvalBinary(v.At(i+k), "==", v.At(j+k))) {
+			cmp := OrderedCompare(c, v.At(i+k), v.At(j+k))
+			if cmp == 0 {
 				continue
 			}
-			return toBool(c.EvalBinary(v.At(i+k), "<", v.At(j+k)))
+			return cmp < 0
 		}
 		return false
 	})

value/vector.go

@@ -387,7 +387,7 @@ func (v *Vector) grade(c Context) *Vector {
 		x[i] = i
 	}
 	sort.SliceStable(x, func(i, j int) bool {
-		return toBool(c.EvalBinary(v.At(x[i]), "<", v.At(x[j])))
+		return OrderedCompare(c, v.At(x[i]), v.At(x[j])) < 0
 	})
 	origin := c.Config().Origin()
 	for i := range x {