Be fully no_std compatible (except for ws rpc)

[haerdib]

Would include hosting a ws client in no-std. To my knowledge, this is not yet available as a library.

[clangenb]

We do not necessarily need a ws-client, just any networking interface that implements the RpcClient or the subscriber.

The cool thing is, some no_std libraries provide their own networking abstractions, like we have it in SGX. Hence, it is already a very useful thing to provide everything in no_std except the ws-client.

As the main part is already generic over the the traits above, we should be able to make almost everything no_std compatible by now.

[haerdib]

I have looked into it a little. Making the api-client no_std compatible is not that much work at all, if one leaves out the ws-client. If #267 is done, I think most of it should be solved already. If one leaves out the ws-client.

Regarding the ws-client:
I haven't found anything (once again) that is no_std even nearly no_std compatible. tungstenite seems to be a fairly simple one, so one could look into that to see how much needs to done to make it no_std compatible. Maybe even a from scratch implementation is necessary. But what's certain: a no_std client will need to be fairly simple and slow. No async, no multithreading, no mio. And a lot of work. Not sure if that's worth it. Maybe a fire-and-forget client is somewhat more reasonable.

Seems to a fairly useful http-client list: https://lib.rs/web-programming/http-client

[clangenb]

Networking does usually need the very low-level primitives that interface with the hardware. I don't actually expect a generic no_std websocket client to be available at all. It is much more likely that the ws_client will be supplied by the user of the library, like our sgx compatible one, or one that is supplied by a browser lib.