add `write_fmt` method to String, to make `write!` macro work without imports

Proposal

Problem statement

It would be great if this code compiled:

fn main() {
    let mut s = String::new();
    write!(s, "hello").unwrap();
}

Using write! to append to a String is one of these neat Rust things that are not obvious to discover, but once you know about it it is amazingly useful.

Solution sketch

The code currently fails saying that io::Write or fmt::Write needs to be imported or a write_fmt method needs to be added. (The right trait to import is fmt::Write, as I found out by trial-and-error.)

From the error message it sounds like that could be avoided if we simply added a write_fmt inherent method to String. That would just get one minor roadblock out of the way which sounds like a win to me. :)

Alternatives

We could decide the roadblock isn't bad enough to warrant doing anything.

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start with the proposal's discussion of String, write!, and fmt::Write, then read the linked API change proposal process and review the 26-comment thread. Done first requires a libs-api decision on the proposed API; if approved, a follow-up implementation path would still need to be identified, since no file or test is named here.