Convenience functions for working with `Formatter` / `PrettyFormatter`

[tgross35]

As far as I can tell, using a custom formatter requires interfacing with Serializer, which means any crate doing this needs the direct serde_core dependency:

use serde_core::Serialize;
use serde_json::{Serializer, json, ser::PrettyFormatter};

fn main() {
    let j = json! {{"a": [1, 2, 3], "b": {}}};
    let p = PrettyFormatter::with_indent(b"    ");
    let mut v = Vec::new();
    let mut s = Serializer::with_formatter(&mut v, p);
    j.serialize(&mut s).unwrap();
    println!("{}", str::from_utf8(&v).unwrap());
}

Would it be possible to add convenience functions like to_{string,vec,writer}_formatter that make this easier?

pub fn to_string_formatter<T, F>(value: &T, formatter: F) -> Result<String>
where
    T: ?Sized + Serialize,
    F: Formatter;

fn main() {
    let j = json! {{"a": [1, 2, 3], "b": {}}};
    let p = PrettyFormatter::with_indent(b"    ");
    println!("{}", serde::to_string_formatter(&j, p).unwrap());
}

Any -> String versions should emit an error if the indentation is not valid UTF-8 (#750).