Replace endian specific copy functions with DataView

[richardlau]

Refs: #13 (comment)

@guybedford This is the tidied up version of the hack I did to get the tests passing on AIX (big endian). I'm okay with #13 so am not fussed if this lands or is just closed out, but I've opened it anyway as an FYI.

FWIW the original hack kept the copyBE and copyLE functions and only used DataView for copyBE. It was hacky because it took me a while to realise that the source length was in characters and DataView's length/offsets were in bytes.

[richardlau]

I also noticed there's a .travis.yml file here -- Travis does have a big endian CPU (s390x) in its beta multiple CPU architecture support: https://docs.travis-ci.com/user/multi-cpu-architectures/

[guybedford]

Thanks @richardlau. I just cloned this and compared it to the performance of the version on master.

I'm getting ~30ms for the Warm runs test on this PR, versus ~28ms for the Warm runs test on average on master.

For this perf reason I think we should stick with what is on master though, at least until DataView is more optimized in Node.js.

[guybedford]

Closing as the other approach is now working well. Thanks again for the help on this one @richardlau.