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/Cargo.toml

@@ -13,12 +13,12 @@ keywords = ["regex", "grep", "egrep", "search", "pattern"]
 license = "Unlicense/MIT"
 
 [dependencies]
+bstr = { version = "0.1.2", default-features = false, features = ["std"] }
 bytecount = "0.5"
 encoding_rs = "0.8.14"
 encoding_rs_io = "0.1.4"
 grep-matcher = { version = "0.1.1", path = "../grep-matcher" }
 log = "0.4.5"
-memchr = "2.1"
 memmap = "0.7"
 
 [dev-dependencies]