Support writing of cross-endian files

[evanmiller]

Currently little-endian machines can only write little-endian files, and big-endian machines can write only big-endian files. Support for cross-endian writes will let us bring cross-endian reads under test coverage.

[QuLogic]

FWIW, the haven test suite passes on s390x, a big endian architecture. I don't know if any of the tests actually exercise endianness, though.

[evanmiller]

@QuLogic Thanks, good to know. The tests all write to their native endianness and read from the same, so there are still untested code paths.

[QuLogic]

Ah, I was hoping some of those existing files in the test suite might have exercised this code path.

[evanmiller]

I should have clarified - the ReadStat tests are all native roundtrips. I believe the haven tests use a suite of files, and therefore will test at least some of the byteswapping code paths on a big-endian machine.

I don't know how comprehensive the haven tests are, however.