let Cursor::peek return span

Summary:
Currently, `CursorBase::peek()` returns `pair<uint8_t const*, size_t>` and `CursorBase::peekView()` returns `basic_string_view<uint8_t>`. Consolidate them into `CursorBase::peek()` returning `span<uint8_t const>`.

As one motivation, libc++ v19 removes `basic_string_view<uint8_t>`.

Reviewed By: thevinster

Differential Revision: D62460851

fbshipit-source-id: 72d567817faf778287594c10dfd7f6f379bf2446

quic/server/QuicServerPacketRouter.cpp

@@ -208,10 +208,10 @@ void TakeoverPacketHandler::processForwardedPacket(
  }
  struct sockaddr* sockaddr = nullptr;
  uint8_t sockaddrBuf[kMaxBufSizeForTakeoverEncapsulation];
- std::pair<const uint8_t*, size_t> addrData = cursor.peek();
- if (addrData.second >= addrLen) {
+ auto addrData = cursor.peek();
+ if (addrData.size() >= addrLen) {
  // the address is contiguous in the queue
- sockaddr = (struct sockaddr*)addrData.first;
+ sockaddr = (struct sockaddr*)addrData.data();
  cursor.skip(addrLen);
  } else {
  // the address is not contiguous, copy it to a local buffer