grep-searcher: add Box<...> impl for Sink

We initially did not have this impl because the first revision of the Sink
trait was much more complicated. In particular, each method was
parameterized over a Matcher. But not every Sink impl actually needs a
Matcher, and it is just as easy to borrow a Matcher explicitly, so the
added parameterization wasn't holding its own.

This does permit Sink implementations to be used as trait objects. One
key use case here is to reduce compile times, since there is quite a bit
of code inside grep-searcher that is parameterized on Sink. Unfortunately,
that code is *also* parameterized on Matcher, and the various printers in
grep-printer are also parameterized on Matcher, which means Sink trait
objects are necessary but no sufficient for a major reduction in compile
times. Unfortunately, the path to making Matcher object safe isn't quite
clear. Extension traits maybe? There's also stuff in the Serde ecosystem
that might help, but the type shenanigans can get pretty gnarly.

grep-searcher/src/sink.rs

@@ -246,6 +246,53 @@ impl<'a, S: Sink> Sink for &'a mut S {
     }
 }
 
+impl<S: Sink + ?Sized> Sink for Box<S> {
+    type Error = S::Error;
+
+    #[inline]
+    fn matched(
+        &mut self,
+        searcher: &Searcher,
+        mat: &SinkMatch,
+    ) -> Result<bool, S::Error> {
+        (**self).matched(searcher, mat)
+    }
+
+    #[inline]
+    fn context(
+        &mut self,
+        searcher: &Searcher,
+        context: &SinkContext,
+    ) -> Result<bool, S::Error> {
+        (**self).context(searcher, context)
+    }
+
+    #[inline]
+    fn context_break(
+        &mut self,
+        searcher: &Searcher,
+    ) -> Result<bool, S::Error> {
+        (**self).context_break(searcher)
+    }
+
+    #[inline]
+    fn begin(
+        &mut self,
+        searcher: &Searcher,
+    ) -> Result<bool, S::Error> {
+        (**self).begin(searcher)
+    }
+
+    #[inline]
+    fn finish(
+        &mut self,
+        searcher: &Searcher,
+        sink_finish: &SinkFinish,
+    ) -> Result<(), S::Error> {
+        (**self).finish(searcher, sink_finish)
+    }
+}
+
 /// Summary data reported at the end of a search.
 ///
 /// This reports data such as the total number of bytes searched and the