Combine CashHolder and SecuritiesHolder into AssetHolderLike

[davidrpugh]

Since both Cash and Securities are really just different kinds of Assets, there should be an AssetHolderLike trait that handles both types.

[davidrpugh]

Want to pass around messages containing Map[AssetLike, Int]. Important that these messages contain immutable maps! Thus need to change the assets field of the AssetHolderLike trait be a var. Note that the hoard, dishoard, accumulate, and deaccumulate methods need to be updated accordingly.