message: Optimise string inclusion
  Summary: Rust likes to spend a lot of time creating factorisations of the string to search for. In many cases we are searching for a single char, so optimise that to memchr.
diff: @@ -264,7 +264,14 @@ impl<'v> StarlarkValue<'v> for StarlarkStr {
  fn is_in(&self, other: Value) -> anyhow::Result<bool> {
  match other.unpack_str() {
  - Some(s) => Ok(self.unpack().contains(s)),
  + Some(s) => {
  + let me = self.unpack();
  + if s.len() == 1 {
  + Ok(me.as_bytes().contains(&s.as_bytes()[0]))
  + } else {
  + Ok(me.contains(s))
  + }
  + }
  None => Err(ValueError::IncorrectParameterType.into()),
  }
  }
  