encode hash endianness in a type

[LarryRuane]

In particular, see block.GetEncodableHash(), block.GetDisplayHash(), and block.GetDisplayPrevHash().

Unfortunately, Satoshi used both big-endian and little-endian extensively, and the reason was not always clear. This has been a source of confusion. This is especially true of block hashes. It may be useful to attach the encoding to the data itself, rather than relying on just convention. This may not be worth the effort.