ripgrep: move minimum version to Rust stable

This also updates some code to make use of our more liberal versioning
requirement, including the use of crossbeam-channel instead of the MsQueue
from the older an unmaintained crossbeam 0.3. This does regrettably add
a sizable number of dependencies, however, compile times seem mostly
unaffected.

Closes #1019

ignore/src/lib.rs

@@ -46,7 +46,7 @@ See the documentation for `WalkBuilder` for many other options.
 
 #![deny(missing_docs)]
 
-extern crate crossbeam;
+extern crate crossbeam_channel as channel;
 extern crate globset;
 #[macro_use]
 extern crate lazy_static;