wishlist: add CT functions #6

More broadly speaking, we want a bunch of CT functions in the standard library:

• addition of CT commitments (need to be careful about asset types)
• multiplication by 64-bit integers (can do up to 19 decimal digits, and can do ratios)
• equality proofs of values
• equality proofs of asset IDs
• rangeproofs
• exact value/asset proofs
• proofs that a value/asset is in a short list

Need to think a bit about how to organize these in the stdlib.

Many of these also need supporting "signer-side" code which can produce the proofs, and which would need to be constant-time etc.

Originally posted by @apoelstra in #5 (comment)