grep-cli: introduce new grep-cli crate

This commit moves a lot of "utility" code from ripgrep core into
grep-cli. Any one of these things might not be worth creating a new
crate, but combining everything together results in a fair number of a
convenience routines that make up a decent sized crate.

There is potentially more we could move into the crate, but much of what
remains in ripgrep core is almost entirely dealing with the number of
flags we support.

In the course of doing moving things to the grep-cli crate, we clean up
a lot of gunk and improve failure modes in a number of cases. In
particular, we've fixed a bug where other processes could deadlock if
they write too much to stderr.

Fixes #990

grep/src/lib.rs

@@ -14,6 +14,7 @@ A cookbook and a guide are planned.
 
 #![deny(missing_docs)]
 
+pub extern crate grep_cli as cli;
 pub extern crate grep_matcher as matcher;
 #[cfg(feature = "pcre2")]
 pub extern crate grep_pcre2 as pcre2;