Add a browser-utils crate

[expenses]

There is a lot of shared logic between the test substrate/polkadot in-browser light clients I've been working on, so I'm putting it in this browser-utils crate.

For clarity: I'm still working out how to best approach certain changes needed to get the light clients to work properly, so I'm merging things piece-by-piece.

[gnunicorn]

Had some minor things, aside from that I don't have any complaints. But I don't know this code or what it should be doing well enough to judge this at all. Inviting @tomaka to take a look instead.

[tomaka]

👍 Python2 is now deprecated 😅

[tomaka]

IMO we should be more explicit in the keystore. The keystore should have a "memory-only" mode rather than using #[cfg] stuff. Once the keystore can be memory-only, we also no longer need to pass a dummy keystore path in the configuration.

What this PR does is okay-ish but doesn't go in the direction of having clean code.

We can either merge now and fix later, or improve this PR itself.

[expenses]

@tomaka What can be cleaned up?

[tomaka]

What I mentioned: not having platform-specific code in the keystore, and the passing of a dummy path for the keystore field in the configuration.

[tomaka]

Well, the next obvious step is to adjust config.keystore_path to be an enum of either a path or "in memory".

[expenses]

👍

[tomaka]

LGTM. Obviously path being an Option is also not great, but I'm not going to ask you to entirely refactor Substrate in this PR.

[tomaka]

This needs a Polkadot PR because of the change in the configuration now though.

[gavofyork]

tests not passing either

[bkchr]

Needs resolving.

[expenses]

Not sure what's causing wasm-bindgen-webidl to not compile - that version seems to work fine on my machine and the build server.