move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/tmap/tmap.go

@@ -0,0 +1,32 @@
+// Package tmap provides type mapping facility that maps type id to type name.
+package tmap
+
+// Map is type mapping.
+type Map struct {
+	types map[uint32]string
+}
+
+func (m *Map) add(mapping map[uint32]string) {
+	for k, v := range mapping {
+		m.types[k] = v
+	}
+}
+
+// Get returns type string or blank.
+func (m *Map) Get(id uint32) string {
+	if m == nil || len(m.types) == 0 {
+		return ""
+	}
+	return m.types[id]
+}
+
+// New creates new Map from mappings.
+func New(mappings ...map[uint32]string) *Map {
+	m := &Map{
+		types: map[uint32]string{},
+	}
+	for _, mapping := range mappings {
+		m.add(mapping)
+	}
+	return m
+}