Support serializing strings containing lone surrogates with deserialize_any

[lucacasonato]

Usually serde_json can deserialize JSON strings into byte buffers, which makes deserializing JSON with lone surrogate pairs (which are valid JSON), possible (see #828).

Unfortunately, this is not possible when serde_json is driving the deserialization visitor, i.e. with deserialize_any. To my knowledge this makes it impossible to deserialize unknown JSON values while not erroring on lone surrogates using serde_json.

I understand this is rather contrived, but it'd be great to figure out a solution to this. I am not really sure how though. My best idea is to add a way for visitors to tell the Deserializer what visitation types it supports/prefers, so the Deserializer can make informed decisions about how to handle data types that can be deserialized in multiple ways in deserialize_any. This could look something like this:

type VisitationType {
  Bool,
  Unsigned,
  Signed,
  Float,
  Char,
  String,
  Bytes,
  Unit,
  Option,
  NewtypeStruct,
  Seq,
  Map,
  Enum,
}

pub trait Visitor<'de>: Sized {
  ...

  /// Hint to the deserializer what types this visitor supports, for when the
  /// deserializer is driving the visitor autonomously using `deserialize_any`.
  ///
  /// This is only a hint. Deserializers are not required to respect the preference.
  fn preferred_types(&self) -> Option<&[VisitationType]> {
    Some(&[])
  }
}

This would require a change to serde though, which may not be desirable or possible.

I'd love to hear feedback on this.