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
This commit does the work to delete the old `grep` crate and effectively
rewrite most of ripgrep core to use the new libripgrep crates. The new
`grep` crate is now a facade that collects the various crates that make
up libripgrep.
The most complex part of ripgrep core is now arguably the translation
between command line parameters and the library options, which is
ultimately where we want to be.
