Retrieve crypto from global instead of Window in WASM

[oliverdunk]

Overview

At 1Password we use ring in our WebExtension, and extensions are currently moving to Manifest V3, where extension code will be run within a service worker. To continue using ring, I developed this patch which allows the WASM code to access crypto from the global without panicking if window is missing (like in a worker).

How to test

1. Clone https://github.com/oliverdunk/wasm-worker (which uses the current version of ring)
2. Run make
3. Open http://localhost:8080/, and see that there is an error in the console
4. Checkout the update-test branch
5. Run make again
6. Open http://localhost:8080/ and see that things ran successfully

Notes

There are a couple of related things that may be worth looking in to in place of this PR as is:

• get_crypto could be cached in a thread_local if that's a performance win. I haven't done any benchmarking there yet but am happy to do so.
• Add optional wasm32_unknown_unknown_getrandom feature #1486 is a super interesting PR and could potentially solve the same problem. We could just provide a custom implementation there.

[briansmith]

See the draft PR #1531 where I propose we delegate the randomness stuff to the getrandom crate. I'm going to close this in favor of that; I'll reopen this if we don't merge that for some reason.