rust: str: add `b_str!` macro

Add the `b_str!` macro, which creates a new `BStr` from
a string literal.

It is usable in const contexts, for instance:

    const X: &BStr = b_str!("Example");

Signed-off-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>