message: Clean up some fixmes in send_streams.rs
  I think we want to use try_from() so unexpected things don't fail silently
  but realistically this will never happen so use an expect() in one case.
  In another case, everything seems to already be a usize, so I don't believe
  the check is needed at all.
diff: @@ -334,13 +334,10 @@ impl TxBuffer {
  // We can drop contig acked range from the buffer
  let new_retirable = self.ranges.acked_from_zero() - self.retired;
  - if let Ok(nr) = usize::try_from(new_retirable) {
  - let keep_len: usize = self.buffered() - nr;
  + let keep_len =
  + self.buffered() - usize::try_from(new_retirable).expect("should fit in usize");
  self.send_buf.truncate_front(keep_len);
  self.retired += new_retirable;
  - } else {
  - // FIXME: What do we want to do here ?
  - }
  }
  pub fn mark_as_lost(&mut self, offset: u64, len: usize) {
  @@ -749,11 +746,7 @@ impl SendStreams {
  pub fn lost(&mut self, token: &StreamRecoveryToken) {
  if let Some(ss) = self.0.get_mut(&token.id) {
  - if let Ok(len) = token.length.try_into() {
  - ss.mark_as_lost(token.offset, len, token.fin);
  - } else {
  - //FIXME: Do we want to log something?
  - }
  + ss.mark_as_lost(token.offset, token.length, token.fin);
  }
  }
  