rust: str: add BStr type

Add the `BStr` type, which is a byte string without UTF-8
validity guarantee.

It is simply an alias to `[u8]`, but has a more evident
semantical meaning.

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

Author: nbdd0121

rust/kernel/str.rs

@@ -4,6 +4,11 @@
 
 use core::fmt;
 
+/// Byte string without UTF-8 validity guarantee.
+///
+/// `BStr` is simply an alias to `[u8]`, but has a more evident semantical meaning.
+pub type BStr = [u8];
+
 /// 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