Fix for new partial-move rules; swap hash le_hex_string and be_hex_string

I noticed that the little/big endian hex string functions for Sha256dHash
did not match my intuition. What we should have is that the raw bytes
correspond to a little-endian representation (since we convert to Uint256
by transmuting, and Uint256's have little-endian representation) while
the reversed raw bytes are big-endian.

This means that the output from `sha256sum` is "little-endian", while the
standard "zeros on the left" output from bitcoind is "big-endian". This
is correct since we think of blockhashes as being "below the target" when
they have lots of zeros on the left, and we also notice that when hashing
Bitcoin objects with sha256sum that the output hashes are always reversed.

These two functions le_hex_string and be_hex_string should really not be
used outside of the library; the Encodable trait should give access to a
"big endian" representation while ConsensusEncodable gives access to a
"little endian" representation. That way we describe the split in terms
of user-facing/consensus code rather than big/little endian code, which
is a better way of thinking about it. After all, a hash is a collection
of bytes, not a number --- it doesn't have an intrinsic endianness.

Oh, and by the way, to compute a sha256d hash from sha256sum, you do

  echo -n 'data' | sha256sum | xxd -r -p | sha256dsum