Replace uWrite impl for String by generic impl for all Write impls

[therealprof]

Alternative version using core::fmt::Error as return type instead of
Infallible.

Signed-off-by: Daniel Egger daniel@eggers-club.de

[japaric]

I agree with the intention of the PR (bridging fmt::Write and uWrite) but I don't think we should use a blanket implementation for that because it makes impossible to implement both uWrite<Error = Infallible> (or some other helpful error type) and fmt::Write for some type at the same time ("overlapping (trait) implementation" rustc error).

Instead we should add an adapter type to ufmt-utils; something along the lines of:

use core::fmt;

pub struct ImBadAtNamingThingsAdapter<W>(pub W) where W: fmt::Write;

impl<W> uWrite for ImBadAtNamingThingsAdapter<W> where W: fmt::Write {
    type Error = fmt::Error;

    // NOTE should also include `write_char`

    fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
        self.0.write_str(s)
    }
}

// then this would work
let mut s = MyStringLikeType::new();
uwrite!(ImBadAtNamingThingsAdapter(&mut s), "{:?}", something_that_impls_udebug);

The uWrite for String implementation should not be removed.

[therealprof]

Sounds good to me.

[therealprof]

How does #21 sound?

[japaric]

Closing in favor of #21