Add Serializer::into_inner method to extract the writer

[Thomas-Mewily]

Hello, this is my first pull request :)
I will try to do my best, don’t hesitate to give me any tips on how I can improve.

Description

This add an into_inner() method to ron::ser::Serializer, allowing users to get back the inner writer.

The name and documentation is the same as the serde_json::Serializer::into_inner().
(it also have the MIT OR Apache-2.0 licence)

Motivation

A typical use case of ron looks like:

  // serializer part : serializer + buffer
  let mut buf = String::new();
  let mut ron_serializer = ron::ser::Serializer::new(&mut buf, None).unwrap();
  
  // The operation that give back the ron representation in buf
  "some structure".serialize(&mut ron_serializer).unwrap();
  println!("{buf}")

However, wrapping this operation into a trait + a 'static structure is impossible without an into_inner():

pub trait MarkupSerializer: Serializer {
    fn extract_markup(self) -> String; // Should be a result, but anyway this is an example
}

impl MarkupSerializer for &mut ron::ser::Serializer<String>
{
    fn extract_markup(self) -> String {
        panic!("impossible without an into_inner")
    }
}

The only way is to also store the buf in the same structure, but that can't be express in rust to my knowledge:

pub struct StringRonSerializer {
    buf: String,
    serializer: ron::Serializer<&'self mut String>, // Imaginary syntax. Reference to Self::buf . Don't compile/Not possible
}

So I don't think storing a ron serializer to progressively serialize somethings, them getting back the serialized string in a lifetimeless / 'static structure is not possible without an into_inner() method.

(My use case is to wrap a ron serializer into another serializer)

[juntyr]

Looks good to me, can you also add a changelog entry?

[Thomas-Mewily]

I added an entry to the changelog, although I'm not sure I put it in the right place.

[juntyr]

The test failures will be fixed in #582