add allocation free versions of lower/upper string
add allocation free versions of lower/upper string

Co-authored-by: LemonBoy <209b8e2fb396024ee6560339c1796e53f9e5a16e@gmail.com>
Co-authored-by: Andrew Kelley <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@ziglang.org>

zig-lang/zig,andrewrk/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig