Change get_url(cachebust=true) to use a hash

[dancek]

Cache-busting was previously done with a compile-time timestamp. Change
to the SHA-256 hash of the file to avoid refreshing unchanged files.

The implementation could be used to add a new global fn (say,
get_file_hash) for subresource integrity use, but that's for another
commit.

Fixes #519.

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[dancek]

I'm not very seasoned with Rust, so I'll appreciate any review comments. Especially the TestContext part seems a bit questionable to me, but this way I think the directory should get cleaned up even when a test panics.

[dancek]

error: failed to run custom build command for `onig_sys v69.2.0`

I don't think this is caused by my changes. 🤔

[Keats]

Don't worry about that, it's being fixed upstream.

[Keats]

Thanks!