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/entity/format.go

@@ -0,0 +1,80 @@
+package entity
+
+import (
+	"strings"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+type utf8entity struct {
+	offset int
+	length int
+}
+
+// Builder builds message string and text entities.
+type Builder struct {
+	entities []tg.MessageEntityClass
+	// lengths stores offset/length of entities too, but in UTF-8 codepoints.
+	lengths []utf8entity
+	// We store index of first entity added at last Format call.
+	// It needed to trim space in all entities of last text block.
+	lastFormatIndex int
+	// utf16length stores length in UTF-16 codepoints.
+	utf16length int
+	// message is message string builder.
+	message strings.Builder
+}
+
+// GrowText grows internal buffer capacity.
+func (b *Builder) GrowText(n int) {
+	b.message.Grow(n)
+}
+
+// GrowEntities grows internal buffer capacity.
+func (b *Builder) GrowEntities(n int) {
+	if n < 0 {
+		panic("entity.Builder.GrowEntities: negative count")
+	}
+
+	buf := make([]tg.MessageEntityClass, len(b.entities), 2*cap(b.entities)+n)
+	copy(buf, b.entities)
+	b.entities = buf
+}
+
+// Reset resets the Builder to be empty.
+func (b *Builder) Reset() {
+	b.message.Reset()
+	b.entities = nil
+	b.utf16length = 0
+}
+
+// UTF8Len returns length of text in bytes.
+func (b *Builder) UTF8Len() int {
+	return b.message.Len()
+}
+
+// UTF16Len returns length of text in UTF-16 codepoints.
+func (b *Builder) UTF16Len() int {
+	return b.utf16length
+}
+
+// EntitiesLen return length of added entities.
+func (b *Builder) EntitiesLen() int {
+	return len(b.entities)
+}
+
+// TextRange returns message text of given byte (UTF-8) range.
+//
+// If range is invalid, it will panic.
+func (b *Builder) TextRange(from, to int) string {
+	return b.message.String()[from:to]
+}
+
+// LastEntity returns last entity if any.
+func (b *Builder) LastEntity() (tg.MessageEntityClass, bool) {
+	l := b.EntitiesLen()
+	if l < 1 {
+		return nil, false
+	}
+	return b.entities[l-1], true
+}