Storage lookup pagination

[ItsNickBarry]

Most of our data structure libraries are represented by arrays internally, and some have a toArray function which returns the underlying data. We need to add a paginated version of toArray which accepts startIndex and length parameters and returns a slice of the underlying array. This will enable partial lookups when a full lookup would be gas-prohibitive.

Apply this to all current and pending data structures (see data-structures branch). Also apply to contracts that use these data structures, such as the Enumerable extensions.