This means node's crypto.randomBytes on the server, and
window.crypto.getRandomValues on the client. If node's crypto.randomBytes throws
an exception, we fall back to crypto.pseudoRandomBytes. If
window.crypto.getRandomValues isn't supported by the browser, we fall back to
the alea generator that we had been using previously.
For repeatable unit test failures with "random" data it's useful to be
able to create deterministic random number sequences.
Introduce `Random.create(seed...)` which returns a object with the
`Random` API (`id()`, `choice()`, etc.) initialized with the passed
seed(s).
