Auto merge of #14351 - asajeffrey:servo-rand-share-fds, r=emilio

An in-memory RNG that shares its file descriptor.

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This PR implements an in-memory random number generator that only uses an OS RNG for (re)seeding. The OS RNG is shared, so there's only one file descriptor for `/dev/urandom` being used.

The PR also implements a tidy check that we don't accidentally introduce an RNG. Rather annoyingly, there are a lot of transitive dependencies on `rand`, notably hash maps in `std`.

This PR makes it possible to use uuids for identifiers such as pipeline and frame ids.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because it's fixing a resource issue

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components/style/Cargo.toml

@@ -44,7 +44,6 @@ owning_ref = "0.2.2"
 parking_lot = "0.3.3"
 phf = "0.7.20"
 quickersort = "2.0.0"
-rand = "0.3"
 rayon = "0.6"
 rustc-serialize = "0.3"
 selectors = "0.15"