Bug 1631721 - Vendor the hashbrown crate r=manishearth

This is the hash map implementation now used in the Rust standard library:

* https://blog.rust-lang.org/2019/07/04/Rust-1.36.0.html#a-new-hashmapk-v-implementation
* rust-lang/rust#58623
* https://crates.io/crates/hashbrown

Differential Revision: https://phabricator.services.mozilla.com/D71740

UltraBlame original commit: 06dcf8ddbfdefed44cb653c7bf533196ae360da3

Author: marco-c

Cargo.lock

@@ -320,6 +320,13 @@ fxhash
 .
 2
 "
+hashbrown
+=
+"
+0
+.
+7
+"
 hashglobe
 =
 {