vm/core: add ID support for SYSCALL, redo interop registration

This solves two problems:
 * adds support for shortened SYSCALL form that uses IDs (similar to #434, but
   for NEO 2.0, supporting both forms), which is important for compatibility
   with C# node and mainnet chain that uses it from some height
 * reworks interop plugging to use callbacks rather than appending to the map,
   these map mangling functions are clearly visible in the VM profiling
   statistics and we want spawning a VM to be fast, so it makes sense
   optimizing it. This change moves most of the work to the init() phase
   making VM setup cheaper.

Caveats:
 * InteropNameToID accepts `[]byte` because that's the thing we have in
   SYSCALL processing and that's the most often usecase for it, it leads to
   some conversions in other places but that's acceptable because those are
   either tests or init()
 * three getInterop functions are: `getDefaultVMInterop`, `getSystemInterop`
   and `getNeoInterop`

Our 100K (1.4M->1.5M) block import time improves by ~4% with this change.

pkg/vm/vm_test.go

@@ -16,12 +16,19 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func fooInteropGetter(id uint32) *InteropFuncPrice {
+	if id == InteropNameToID([]byte("foo")) {
+		return &InteropFuncPrice{func(evm *VM) error {
+			evm.Estack().PushVal(1)
+			return nil
+		}, 1}
+	}
+	return nil
+}
+
 func TestInteropHook(t *testing.T) {
 	v := New()
-	v.RegisterInteropFunc("foo", func(evm *VM) error {
-		evm.Estack().PushVal(1)
-		return nil
-	}, 1)
+	v.RegisterInteropGetter(fooInteropGetter)
 
 	buf := new(bytes.Buffer)
 	EmitSyscall(buf, "foo")
@@ -32,13 +39,27 @@ func TestInteropHook(t *testing.T) {
 	assert.Equal(t, big.NewInt(1), v.estack.Pop().value.Value())
 }
 
-func TestRegisterInterop(t *testing.T) {
+func TestInteropHookViaID(t *testing.T) {
 	v := New()
-	currRegistered := len(v.interop)
-	v.RegisterInteropFunc("foo", func(evm *VM) error { return nil }, 1)
-	assert.Equal(t, currRegistered+1, len(v.interop))
-	_, ok := v.interop["foo"]
-	assert.Equal(t, true, ok)
+	v.RegisterInteropGetter(fooInteropGetter)
+
+	buf := new(bytes.Buffer)
+	fooid := InteropNameToID([]byte("foo"))
+	var id = make([]byte, 4)
+	binary.LittleEndian.PutUint32(id, fooid)
+	_ = EmitSyscall(buf, string(id))
+	_ = EmitOpcode(buf, opcode.RET)
+	v.Load(buf.Bytes())
+	runVM(t, v)
+	assert.Equal(t, 1, v.estack.Len())
+	assert.Equal(t, big.NewInt(1), v.estack.Pop().value.Value())
+}
+
+func TestRegisterInteropGetter(t *testing.T) {
+	v := New()
+	currRegistered := len(v.getInterop)
+	v.RegisterInteropGetter(fooInteropGetter)
+	assert.Equal(t, currRegistered+1, len(v.getInterop))
 }
 
 func TestBytesToPublicKey(t *testing.T) {