Restructure library.jl

[tlnagy]

Due to some poor designing on my part, the Library type in library.jl is monolithic and fragile. It would be much better to have this as two separate types, Genome and Library. Then this new slimmed down Library type could be integrated with the already existing Cas9behavior type. Separately, num_genes and num_guides should be moved from the Screen types into Genome.

Additionally, instead of the hackish thing I'm doing here: https://github.com/tlnagy/pooled-screen-optimization/blob/41ff08519ad9775733c26552d43e1649d0962008/src/simulation/library.jl#L68-L83 with the combination of dictionaries and categorical distributions, I could use Types to encompass this information and make the whole thing cleaner.