showcrcs: util to write out crc information from a bzip2 file

The new utility can also compute the file crc from the block crcs,
which would let us write a new file with arbitrary blocks from
other bzip2 files, write the new streamcrc, and never uncompress
the data.

[WIP] need to test against files with tiny blocks in them
(eg headers and footers), and check all other utils that were
rebuilt after the library change

Bug: T216009
Change-Id: Ie6372db0851872b7b61968a3a809b81148d891b0