matcher: polish the grep-matcher crate

Not much here. Just updating to reflect my current style and bringing
the crate to the 2021 edition.

crates/matcher/tests/test_matcher.rs

@@ -1,5 +1,7 @@
-use grep_matcher::{Captures, Match, Matcher};
-use regex::bytes::Regex;
+use {
+    grep_matcher::{Captures, Match, Matcher},
+    regex::bytes::Regex,
+};
 
 use crate::util::{RegexMatcher, RegexMatcherNoCaps};