assert for too many shards

[burdges]

We need an assert that prevents dong too many shards: paritytech/polkadot#6308 (comment)

[ordian]

We already have it paritytech/polkadot#6308 (comment)
Btw, the dependency we're using in in polkadot was published from https://github.com/paritytech/reed-solomon-novelpoly/.

[burdges]

Yeah I was just being stupid.. lol

[burdges]

I know it lives elsewhere, this is the fork where we envision eventually optimizing the arithmetic, but everyone is busy with other stuff.