Add a structured representation of an SDMF share

See tahoe-lafs/docs/specifications/mutable.rst, around line 575, the section titled "SDMF Slot Format".

Define a type that can represent all of these fields.

mentioned in issue #4 (closed)

mentioned in issue #5 (closed)

changed milestone to %Mobile: Arbitrary N/k SDMF

mentioned in merge request !2 (merged)

Fixed by !2 (merged)

closed