Convert Buffers, not Byte Arrays

This pull request changes the underlying types used by `convert` to use `Buffer` instead of byte arrays.
This pulls it into line with the rest of the code base, and removes some pointless conversions.

It also removes `convert.reverseEndian`, and adds more tests (and respective edge cases) to the `convert` tests.
Lastly, it fixes 2 incorrect (`undefined`) test descriptions in `Address` and `HDNode`.