Notify multicore<->WASM compatibility issues

[CPerezz]

This PR mainly includes 2 things:

1. A check (which I'm still not sure about) that will force the compilation to terminate if multicore feature is active while we compile to WASM.
2. A README section that explains the issue and leaves it clear that these two things can't work (yet) together.

This should close #120 & #116

[han0110]

LGTM! Just have a small question

[CPerezz]

@huitseeker @han0110 what about merging #122 instead of this? Feels much more clean.

[huitseeker]

@CPerezz it is, but the feature removal makes it a new minor version bump (to 0.6.0) whereas the present one can be issued with just a point release (0.5.1). I'll take #122 for a whirl on Arecibo and see if it works, and report here.

[CPerezz]

I'll take #122 for a whirl on Arecibo and see if it works, and report here.

That's lovely! Will do the same for Halo2. Also, happy to release both anyways. Such that we have both flavours for now.

[huitseeker]

@CPerezz Tested on Arecibo: found a few warts we have to correct on our side, no issues with #122 that I could find.

[CPerezz]

I also tested with privacy-scaling-explorations/halo2#242 and works perfect. Just need some fixes on our side. But nothing wrong with the PR.

So I'm tempted to close this and not merge it.
Do you need it @huitseeker ?

[CPerezz]

Solved via #122