ripgrep: migrate to libripgrep

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.

src/preprocessor.rs

@@ -3,8 +3,6 @@ use std::io::{self, Read};
 use std::path::{Path, PathBuf};
 use std::process::{self, Stdio};
 
-use Result;
-
 /// PreprocessorReader provides an `io::Read` impl to read kids output.
 #[derive(Debug)]
 pub struct PreprocessorReader {
@@ -26,7 +24,7 @@ impl PreprocessorReader {
     pub fn from_cmd_path(
         cmd: PathBuf,
         path: &Path,
-    ) -> Result<PreprocessorReader> {
+    ) -> io::Result<PreprocessorReader> {
         let child = process::Command::new(&cmd)
             .arg(path)
             .stdin(Stdio::from(File::open(path)?))
@@ -34,10 +32,13 @@ impl PreprocessorReader {
             .stderr(Stdio::piped())
             .spawn()
             .map_err(|err| {
-                format!(
-                    "error running preprocessor command '{}': {}",
-                    cmd.display(),
-                    err,
+                io::Error::new(
+                    io::ErrorKind::Other,
+                    format!(
+                        "error running preprocessor command '{}': {}",
+                        cmd.display(),
+                        err,
+                    ),
                 )
             })?;
         Ok(PreprocessorReader {