Added to_string_mut and to_string_mut_pretty

[JohnScience]

/// Serialize the given data structure as JSON into the pre-existing [`String``].
///
/// # Errors
///
/// Serialization can fail if `T`'s implementation of `Serialize` decides to
/// fail, or if `T` contains a map with non-string keys.
#[inline]
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub fn to_string_mut<T>(s: &mut String, value: &T) -> Result<()>
where
    T: ?Sized + Serialize,
{
    // to_writer() guarantees that it feeds only valid UTF-8 to the writer.
    let buf = unsafe { s.as_mut_vec() };
    let mut writer = std::io::Cursor::new(buf);
    to_writer(&mut writer, value)
}

/// Serialize the given data structure as pretty-printed JSON into the pre-existing [`String`].
///
/// # Errors
///
/// Serialization can fail if `T`'s implementation of `Serialize` decides to
/// fail, or if `T` contains a map with non-string keys.
#[inline]
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub fn to_string_mut_pretty<T>(s: &mut String, value: &T) -> Result<()>
where
    T: ?Sized + Serialize,
{
    // to_writer_pretty() guarantees that it feeds only valid UTF-8 to the writer.
    let buf = unsafe { s.as_mut_vec() };
    let mut writer = std::io::Cursor::new(buf);
    to_writer_pretty(&mut writer, value)
}

[JohnScience]

Ready for the review!

[dtolnay]

#1122 describes some considerations about exposing no-std to_writer. What's done in this PR is not going to work.