trimmedSRS optimization

[mimoo]

In the PCD papers they always define their PCS with a trim function, which if I remember correctly works like this:

let n = closest_power_of_2(domain_size);
generators = srs.generators[:n]

where closest_power_of_2 returns 4 if your domain size if 3 or 4, 8 if your domain size is 5, 6, 7, or 8, etc.

@imeckler had pushed a PR here: #134 which we never merged for reasons that are unclear.

It sounds like this would make proving much faster for small circuits when we share the URS (e.g. in Mina), but it's not clear how this would be compatible with recursion.