QDTypes learn to_bits and from_bits

[tanujkhattar]

This is useful to support classical and tensor simulations and support conversion of control values to Cirq. I'll demonstrate usage in follow-up PRs.

cc @fdmalone

[fdmalone]

Very nice. Some questions / comments. I think we need to consolidate bit_iter tools (although this could be a follow up). The qualtran tools do restrict integers to be less than 64 bits, I'm not sure how fundamental that is.

[tanujkhattar]

@fdmalone I've addressed all your comments, PTAL!

[fdmalone]

I like this.