libripgrep: initial commit introducing libripgrep

libripgrep is not any one library, but rather, a collection of libraries
that roughly separate the following key distinct phases in a grep
implementation:

  1. Pattern matching (e.g., by a regex engine).
  2. Searching a file using a pattern matcher.
  3. Printing results.

Ultimately, both (1) and (3) are defined by de-coupled interfaces, of
which there may be multiple implementations. Namely, (1) is satisfied by
the `Matcher` trait in the `grep-matcher` crate and (3) is satisfied by
the `Sink` trait in the `grep2` crate. The searcher (2) ties everything
together and finds results using a matcher and reports those results
using a `Sink` implementation.

Closes #162

grep-pcre2/src/lib.rs

@@ -0,0 +1,15 @@
+/*!
+An implementation of `grep-matcher`'s `Matcher` trait for
+[PCRE2](https://www.pcre.org/).
+*/
+
+#![deny(missing_docs)]
+
+extern crate grep_matcher;
+extern crate pcre2;
+
+pub use error::{Error, ErrorKind};
+pub use matcher::{RegexCaptures, RegexMatcher, RegexMatcherBuilder};
+
+mod error;
+mod matcher;