compiler: support var-declaration of byte slices

Related #801.

pkg/compiler/codegen.go

@@ -235,7 +235,8 @@ func (c *codegen) emitDefault(t types.Type) {
 		if isCompoundSlice(t) {
 			emit.Opcode(c.prog.BinWriter, opcode.NEWARRAY0)
 		} else {
-			emit.Bytes(c.prog.BinWriter, []byte{})
+			emit.Int(c.prog.BinWriter, 0)
+			emit.Opcode(c.prog.BinWriter, opcode.NEWBUFFER)
 		}
 	case *types.Struct:
 		emit.Int(c.prog.BinWriter, int64(t.NumFields()))

pkg/compiler/slice_test.go

@@ -201,6 +201,17 @@ var sliceTestCases = []testCase{
 		}`,
 		[]byte{0x61, 0x62, 0x63},
 	},
+	{
+		"declare and append byte-slice",
+		`package foo
+		func Main() []byte {
+			var a []byte
+			a = append(a, 1)
+			a = append(a, 2)
+			return a
+		}`,
+		[]byte{1, 2},
+	},
 }
 
 func TestSliceOperations(t *testing.T) {