Add WASM CI checks and make availability-store compile for WASM

[tomaka]

cc #625

The change to availability-store is kind of the Polkadot equivalent to paritytech/substrate#3957, except that we enforce this at compile time.

In order to prevent this from breaking accidentally, also adds CI checks.

[tomaka]

I copy-pasted these lines but I have no idea what they are.

[tomaka]

Oh. Interestingly, running cargo build --package foo fails whereas doing cd foo && cargo build works. Will investigate more.

[rphmeier]

Why does the availability store need to compile to WASM?

[tomaka]

Why does the availability store need to compile to WASM?

We need some conditional compilation somewhere, and where is a question of surprise factor/usability/cleanness.

Either we make availability-store compile for WASM, or we have to tweak the node in order to not compile or use availability-store.
Since it's going to be the same for other crates, the code of the node could become quite messy and difficult to read if all the conditional compilation happens at a single place.

[rphmeier]

I don't follow, do you mean that this should be used in the runtime? Because especially after #345 that won't be possible

[tomaka]

Ah no, this PR is about wasm-in-the-browser! Sorry for the confusion.