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/crypto/sha256.go

@@ -0,0 +1,29 @@
+package crypto
+
+import (
+	"crypto/sha256"
+	"hash"
+	"sync"
+)
+
+// SHA256 returns SHA256 hash.
+func SHA256(from ...[]byte) []byte {
+	h := getSHA256()
+	defer sha256Pool.Put(h)
+	for _, b := range from {
+		_, _ = h.Write(b)
+	}
+	return h.Sum(nil)
+}
+
+var sha256Pool = &sync.Pool{
+	New: func() interface{} {
+		return sha256.New()
+	},
+}
+
+func getSHA256() hash.Hash {
+	h := sha256Pool.Get().(hash.Hash)
+	h.Reset()
+	return h
+}