Add a `bformat!` macro

[samvv]

In the light of GitoxideLabs/gitoxide#2139 I was wondering if it is possible to add a bformat! library macro to this crate that does much the same thing as format! but for BStrings.

I've found this occasionally useful to have, for example in the aforementioned issue to map the path of a URL from foobar to foobar.git.

[BurntSushi]

I thought there was an issue for this, but I guess not.

I do think this would be nice to have.

I'm not sure it's likely to happen any time soon. There's probably a fair amount of API design required and implementation effort. The formatting machinery in std is a big beast, and it's not really something I'm looking to add to my maintenance burden. We could start with a smaller subset, but it's one of those things where it's inevitable that folks will just keep asking for features until it matches what std provides.