blocks portion of the segment summary.
A segment summary block begins with a header (SEGSUM); the rest of the
block contains FINFO structures describing file blocks growing upward
from the bottom (after the header), and IINFO structures describing
inode blocks grown downward from the end of the block. (When they meet
the segment is full regardless of how many blocks might be left.)
IINFO contains just a block number, and until now this information was
handled by just using uint32_t*; switching to a structure will make
the code a lot easier to read, and also make it easier to have 32-bit
and 64-bit versions without making a mess.
This commit just adds the structures and accessors; they'll be
deployed into the code in subsequent commits.
8f0c85a18e