Support non-moving segements and other large block groups

Block groups on the haskell heap normally only consist of one block,
but can be longer, eg, for large/pinned objects, compact regions, or for
segments on the nonmoving heap.

We have to take care in our implementation of the block cache and take
this into account when looking up closures.

See: Note [Block groups and the block cache].

Resolves #7