Add 'uncompressed' file backend

The default (snappy) compression is good for fast compression and
decompression for streaming to and from disk, but it is rather poor as a
archival or transmission format.

Adding an uncompressed file backend allows external compression programs
to be used more optimally, rather than struggling with the
relatively-uncompressable output from snappy.

Example trace:
1362mb default (snappy)
3360mb uncompressed

Then compressed with 'xz -6':
769mb default (snappy)
471mb uncompressed
Add compression option to 'apitrace repack'

Allows conversion between different compression formats

--compression/-c options:
'none': No compression used
'snappy': Snappy compression used (default)
'zlib': ZLib compression used (not supported for random seeking)