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>

rust/kernel/str.rs

@@ -9,6 +9,27 @@
 /// `BStr` is simply an alias to `[u8]`, but has a more evident semantical meaning.
 pub type BStr = [u8];
 
+/// Creates a new [`BStr`] from a string literal.
+///
+/// `b_str!` converts the supplied string literal to byte string, so non-ASCII
+/// characters can be included.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::b_str;
+/// # use kernel::str::BStr;
+/// const MY_BSTR: &BStr = b_str!("My awesome BStr!");
+/// ```
+#[macro_export]
+macro_rules! b_str {
+    ($str:literal) => {{
+        const S: &'static str = $str;
+        const C: &'static $crate::str::BStr = S.as_bytes();
+        C
+    }};
+}
+
 /// Allows formatting of [`fmt::Arguments`] into a raw buffer.
 ///
 /// It does not fail if callers write past the end of the buffer so that they can calculate the