Allow writing to &mut dyn WriteStyle

[dtolnay]

Example:

use codespan_reporting::diagnostic::{Diagnostic, Label};
use codespan_reporting::files::SimpleFiles;
use codespan_reporting::term::termcolor::{ColorChoice, StandardStream};
use codespan_reporting::term::{self, Config, WriteStyle};

fn main() {
    let writer = StandardStream::stderr(ColorChoice::Auto);
    repro(&mut writer.lock());
}

fn repro(stderr: &mut dyn WriteStyle) {
    let mut files = SimpleFiles::new();
    let file = files.add("-", "...");

    let mut diagnostic = Diagnostic::error().with_message("...");
    diagnostic.labels.push(Label {
        message: "...".to_owned(),
        ..Label::primary(file, 1..2)
    });

    let config = Config::default();
    let _ = term::emit_to_write_style(stderr, &config, &files, &diagnostic);
}

This used to work in codespan-reporting 0.12.0 (with 2 minor substitutions, term::emit_to_write_style -> term::emit and term::WriteStyle -> term::termcolor::WriteColor) but fails to compile in codespan-reporting 0.13.0 due to missing ?Sized bounds.

error[E0277]: the size for values of type `dyn WriteStyle` cannot be known at compilation time
  --> src/main.rs:22:39
   |
22 |     let _ = term::emit_to_write_style(stderr, &config, &files, &diagnostic);
   |             ------------------------- ^^^^^^ doesn't have a size known at compile-time
   |             |
   |             required by a bound introduced by this call
   |
   = help: the trait `Sized` is not implemented for `dyn WriteStyle`
note: required by an implicit `Sized` bound in `emit_to_write_style`
  --> $CARGO_HOME/registry/src/index.crates.io-1949cf8c6b5b557f/codespan-reporting-0.13.0/src/term/mod.rs:80:63
   |
80 | pub fn emit_to_write_style<'files, F: Files<'files> + ?Sized, W: WriteStyle>(
   |                                                               ^ required by the implicit `Sized` requirement on this type parameter in `emit_to_write_style`