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/bin/string.go

@@ -0,0 +1,60 @@
+package bin
+
+import "io"
+
+const (
+	// If L <= 253, the serialization contains one byte with the value of L,
+	// then L bytes of the string followed by 0 to 3 characters containing 0,
+	// such that the overall length of the value be divisible by 4,
+	// whereupon all of this is interpreted as a sequence of int(L/4)+1 32-bit numbers.
+	maxSmallStringLength = 253
+	// If L >= 254, the serialization contains byte 254, followed by 3 bytes with
+	// the string length L, followed by L bytes of the string, further followed
+	// by 0 to 3 null padding bytes.
+	firstLongStringByte = 254
+)
+
+func encodeString(b []byte, v string) []byte {
+	l := len(v)
+	if l <= maxSmallStringLength {
+		b = append(b, byte(l))
+		b = append(b, v...)
+		currentLen := l + 1
+		b = append(b, make([]byte, nearestPaddedValueLength(currentLen)-currentLen)...)
+		return b
+	}
+
+	b = append(b, firstLongStringByte, byte(l), byte(l>>8), byte(l>>16))
+	b = append(b, v...)
+	currentLen := l + 4
+	b = append(b, make([]byte, nearestPaddedValueLength(currentLen)-currentLen)...)
+
+	return b
+}
+
+func decodeString(b []byte) (padding int, v string, err error) {
+	if len(b) == 0 {
+		return 0, "", io.ErrUnexpectedEOF
+	}
+	if b[0] == firstLongStringByte {
+		if len(b) < 4 {
+			return 0, "", io.ErrUnexpectedEOF
+		}
+		strLen := uint32(b[1]) | uint32(b[2])<<8 | uint32(b[3])<<16
+		if len(b) < (int(strLen) + 4) {
+			return 0, "", io.ErrUnexpectedEOF
+		}
+		return nearestPaddedValueLength(int(strLen) + 4), string(b[4 : strLen+4]), nil
+	}
+	strLen := int(b[0])
+	if len(b) < (strLen + 1) {
+		return 0, "", io.ErrUnexpectedEOF
+	}
+	if strLen > maxSmallStringLength {
+		return 0, "", &InvalidLengthError{
+			Length: strLen,
+			Where:  "string",
+		}
+	}
+	return nearestPaddedValueLength(strLen + 1), string(b[1 : strLen+1]), nil
+}