ivy: fix matrix index of matrix (#112)

The right-to-left evaluation order for indexes was causing the
accumulation of the shape to end up reversed. Fix that by doing
an additional reverse after the loop.

Fixes #111.

testdata/binary_matrix.ivy

@@ -818,3 +818,14 @@ x[down x]
 	7 8 9
 	4 5 6
 	1 2 3
+
+(5 5 rho iota 25)[3]
+	11 12 13 14 15
+
+(5 5 rho iota 25)[3 2]
+	11 12 13 14 15
+	 6  7  8  9 10
+
+(5 5 rho iota 25)[3 2; 1 2 3]
+	11 12 13
+	 6  7  8

value/index.go

@@ -40,7 +40,11 @@ func (ix *indexState) init(context Context, top, left Expr, index []Expr) {
 			ix.xshape = append(ix.xshape, len(x))
 		case *Matrix:
 			ix.indexes[i] = x.Data()
-			ix.xshape = append(ix.xshape, x.Shape()...)
+			// Append shape in reverse, because ix.shape will be reversed below.
+			shape := x.Shape()
+			for j := len(shape) - 1; j >= 0; j-- {
+				ix.xshape = append(ix.xshape, shape[j])
+			}
 		}
 		for _, v := range ix.indexes[i] {
 			if _, ok := v.(Int); !ok {
@@ -49,6 +53,11 @@ func (ix *indexState) init(context Context, top, left Expr, index []Expr) {
 		}
 	}
 
+	// Walked indexes right-to-left, so reverse shape.
+	for i, j := 0, len(ix.xshape)-1; i < j; i, j = i+1, j-1 {
+		ix.xshape[i], ix.xshape[j] = ix.xshape[j], ix.xshape[i]
+	}
+
 	// Can now safely evaluate left side
 	// (must wait until indexes have been evaluated, R-to-L).
 	ix.lhs = left.Eval(context)