4.X: US State with Subsections - MD, KY, RI, IN

[yuzawa-san]

i came up with this independently to #104 which does not have support for the subsection headers per the spec (at least how i understand it). i came up with this before the spec was finalized but it appears to us the semantics i expected, specifically around the encoding of subsections. i had previously conveyed my concern around how it was not possible to encode the core section with subsection id 0 in the fibonacci string since they do not support encoding 0. given that the spec addresses that:

List of subsection ids that are contained in this section. The IDs must be represented in the order the related subsections appear in the string. The Core subsection is required and always included first, so its ID (zero) is not included in the subsection IDs list.

this introduces a new section subclass which automates this type of state schema
there is no longer need to unencodable booleans since the presence was in the header.