searcher: partially migrate to bstr

This commit causes grep-searcher to use byte strings internally for its
line buffer support. We manage to remove a use of `unsafe` by doing this
(by pushing it down into `bstr`).

We stop short of using byte strings everywhere else because we rely
heavily on the `impl ops::Index<[u8]> for grep_matcher::Match` impl,
which isn't available for byte strings. (It is premature to make bstr a
public dep of a core crate like grep-matcher, but maybe some day.)

grep-searcher/src/lib.rs

@@ -99,13 +99,13 @@ searches stdin.
 
 #![deny(missing_docs)]
 
+extern crate bstr;
 extern crate bytecount;
 extern crate encoding_rs;
 extern crate encoding_rs_io;
 extern crate grep_matcher;
 #[macro_use]
 extern crate log;
-extern crate memchr;
 extern crate memmap;
 #[cfg(test)]
 extern crate regex;