compiler: support removing slice elements

Go-way of removing elements from slice is via `append` builtin.
There is a separate opcode for removing elements from
Arrays, which is cheaper and supported in this commit.

pkg/compiler/analysis.go

@@ -17,7 +17,7 @@ var (
 	goBuiltins = []string{"len", "append", "panic", "make", "copy", "recover", "delete"}
 	// Custom builtin utility functions.
 	customBuiltins = []string{
-		"FromAddress", "Equals",
+		"FromAddress", "Equals", "Remove",
 		"ToBool", "ToByteArray", "ToInteger",
 	}
 )

pkg/compiler/codegen.go

@@ -1542,6 +1542,12 @@ func (c *codegen) convertBuiltin(expr *ast.CallExpr) {
 			typ = stackitem.BooleanT
 		}
 		c.emitConvert(typ)
+	case "Remove":
+		if !isCompoundSlice(c.typeOf(expr.Args[0])) {
+			c.prog.Err = errors.New("`Remove` supports only non-byte slices")
+			return
+		}
+		emit.Opcode(c.prog.BinWriter, opcode.REMOVE)
 	case "Equals":
 		emit.Opcode(c.prog.BinWriter, opcode.EQUAL)
 	case "FromAddress":

pkg/compiler/slice_test.go

@@ -327,6 +327,30 @@ func TestSubsliceCompound(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestRemove(t *testing.T) {
+	t.Run("Valid", func(t *testing.T) {
+		src := `package foo
+		import "github.com/nspcc-dev/neo-go/pkg/interop/util"
+		func Main() int {
+			a := []int{11, 22, 33}
+			util.Remove(a, 1)
+			return len(a) + a[0] + a[1]
+		}`
+		eval(t, src, big.NewInt(46))
+	})
+	t.Run("ByteSlice", func(t *testing.T) {
+		src := `package foo
+		import "github.com/nspcc-dev/neo-go/pkg/interop/util"
+		func Main() int {
+			a := []byte{11, 22, 33}
+			util.Remove(a, 1)
+			return len(a)
+		}`
+		_, err := compiler.Compile("", strings.NewReader(src))
+		require.Error(t, err)
+	})
+}
+
 func TestJumps(t *testing.T) {
 	src := `
 	package foo

pkg/interop/util/util.go

@@ -17,3 +17,8 @@ func FromAddress(address string) []byte {
 func Equals(a, b interface{}) bool {
 	return false
 }
+
+// Remove removes element with index i from slice.
+// This is done in place and slice must have type other than `[]byte`.
+func Remove(slice interface{}, i int) {
+}