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/telegram/message/html/unescape.go

@@ -0,0 +1,129 @@
+package html
+
+import "unicode/utf8"
+
+// unescapeEntity reads an entity like "<" from b[src:] and writes the
+// corresponding "<" to b[dst:], returning the incremented dst and src cursors.
+// Precondition: b[src] == '&' && dst <= src.
+//
+// This is adaption of html.UnescapeString from Go sources.
+func unescapeEntity(b []byte, dst, src int) (dst1, src1 int) {
+	// i starts at 1 because we already know that s[0] == '&'.
+	i, s := 1, b[src:]
+
+	if len(s) <= 1 {
+		b[dst] = b[src]
+		return dst + 1, src + 1
+	}
+
+	if s[i] == '#' {
+		if len(s) <= 3 { // We need to have at least "&#.".
+			b[dst] = b[src]
+			return dst + 1, src + 1
+		}
+		i++
+		c := s[i]
+		hex := false
+		if c == 'x' || c == 'X' {
+			hex = true
+			i++
+		}
+
+		x := '\x00'
+		for i < len(s) {
+			c = s[i]
+			i++
+			if hex {
+				switch {
+				case '0' <= c && c <= '9':
+					x = 16*x + rune(c) - '0'
+					continue
+				case 'a' <= c && c <= 'f':
+					x = 16*x + rune(c) - 'a' + 10
+					continue
+				case 'A' <= c && c <= 'F':
+					x = 16*x + rune(c) - 'A' + 10
+					continue
+				}
+			} else if '0' <= c && c <= '9' {
+				x = 10*x + rune(c) - '0'
+				continue
+			}
+			if c != ';' {
+				i--
+			}
+			break
+		}
+
+		if i <= 3 { // No characters matched.
+			b[dst] = b[src]
+			return dst + 1, src + 1
+		}
+
+		if x == 0 || x >= 0x10ffff {
+			b[dst] = b[src]
+			return dst + 1, src + 1
+		}
+
+		return dst + utf8.EncodeRune(b[dst:], x), src + i
+	}
+
+	// Consume the maximum number of characters possible, with the
+	// consumed characters matching one of the named references.
+
+	for i < len(s) {
+		c := s[i]
+		i++
+		// Lower-cased characters are more common in entities, so we check for them first.
+		if 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || '0' <= c && c <= '9' {
+			continue
+		}
+		if c != ';' {
+			i--
+		}
+		break
+	}
+
+	var x rune
+	tagEnd := i
+	if i > 0 && s[tagEnd-1] == ';' {
+		tagEnd--
+	}
+	switch string(s[1:tagEnd]) {
+	case "lt":
+		x = '<'
+	case "gt":
+		x = '>'
+	case "amp":
+		x = '&'
+	case "quot":
+		x = '"'
+	}
+	if x != 0 {
+		return dst + utf8.EncodeRune(b[dst:], x), src + i
+	}
+
+	dst1, src1 = dst+i, src+i
+	copy(b[dst:dst1], b[src:src1])
+	return dst1, src1
+}
+
+// telegramEscape implements Telegram BotAPI HTML unescape.
+func telegramUnescape(b []byte) []byte {
+	for i, c := range b {
+		if c == '&' {
+			dst, src := unescapeEntity(b, i, i)
+			for src < len(b) {
+				c := b[src]
+				if c == '&' {
+					dst, src = unescapeEntity(b, dst, src)
+				} else {
+					b[dst] = c
+					dst, src = dst+1, src+1
+				}
+			}
+			return b[0:dst]
+		}
+	}
+	return b
+}