This changes how ripgrep emit exit status codes. In particular, any error
that occurs while searching will now cause ripgrep to emit a `2` exit
code, where as it previously would emit either a `0` or a `1` code based
on whether it matched or not. That is, ripgrep would only emit a `2` exit
code for a catastrophic error.
This tweak includes additional logic that GNU grep adheres to, which seems
like good sense. Namely, if -q/--quiet is given, and an error occurs and
a match occurs, then ripgrep will emit a `0` exit code.
Closes#1159
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.
