WASM support

[Erik1000]

Sequoia-openpgp (one of our biggest dependencies) introduced wasm support in 1.7. Now, the only blocker is dtolnay/typetag#8. Sadly, this is a huge problem which stretches all up to a (pre-) RFC for the rust compiler itself and it does not seem like there's any work being done. This leaves us with basically twp possibilities:

• dont use traits in serde. Instead, an enum could work, but it would be really ugly
• implement this (big task)

I personally have no knowledge nor time to acquire it and therefore can't implement this on my own. If you need motivation, keep in mind that this is a big blocker for many people and they all would be really thankful for your work.