compiler: emit Buffer for byte slices

All byte slices are mutable so buffer is a right stack item.

pkg/compiler/slice_test.go

@@ -180,6 +180,27 @@ var sliceTestCases = []testCase{
 			vm.NewByteArrayItem([]byte("b")),
 		},
 	},
+	{
+		"byte-slice assignment",
+		`package foo
+		func Main() []byte {
+			a := []byte{0, 1, 2}
+			a[1] = 42
+			return a
+		}`,
+		[]byte{0, 42, 2},
+	},
+	{
+		"byte-slice assignment after string conversion",
+		`package foo
+		func Main() []byte {
+			a := "abc"
+			b := []byte(a)
+			b[1] = 42
+			return []byte(a)
+		}`,
+		[]byte{0x61, 0x62, 0x63},
+	},
 }
 
 func TestSliceOperations(t *testing.T) {