globset: allow building globs from OS strings

[BurntSushi]

Initially reported here: rust-lang/glob#78

While globset can match on arbitrary OS strings, it cannot accept an arbitrary OS string as a glob. The link above gives an example. Here it is in code form (which currently prints false):

use std::error::Error;
use std::ffi::OsStr;
use std::os::unix::ffi::OsStrExt;

use globset::Glob;

fn main() -> Result<(), Box<dyn Error>> {
    let glob = Glob::new(r"d\xE9cembre 2018 - *.jpg")?;
    let matcher = glob.compile_matcher();
    let filename = OsStr::from_bytes(b"d\xE9cembre 2018 - foo.jpg");
    println!("{:?}", matcher.is_match(filename));
    Ok(())
}

To be clear, this particular program shouldn't succeed because a UTF-8 string is given to Glob::new. But there should be a way to, for example, call Glob::new(some_os_str). On Unix, that OS string can be arbitrary bytes.

(The globset crate is due for an API refresh and likely a rewrite of its internal parser anyway. This should be fairly easy to support.)