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/Cargo.toml

@@ -18,7 +18,7 @@ name = "ignore"
 bench = false
 
 [dependencies]
-crossbeam = "0.3"
+crossbeam-channel = "0.2"
 globset = { version = "0.4.0", path = "../globset" }
 lazy_static = "1"
 log = "0.4"