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/ascii/ascii.go

@@ -0,0 +1,19 @@
+// Package ascii provides data and functions to test some properties of
+// ASCII code points.
+package ascii
+
+// IsDigit reports whether the rune is a decimal digit.
+func IsDigit(r rune) bool {
+	return r >= '0' && r <= '9'
+}
+
+// IsLatinLetter reports whether the rune is ASCII latin letter.
+func IsLatinLetter(r rune) bool {
+	return (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z')
+}
+
+// IsLatinLower reports whether the rune is lower ASCII latin letter.
+func IsLatinLower(r rune) bool {
+	r |= 0x20
+	return 'a' <= r && r <= 'z'
+}