[dev] Change Digest hashing implementation

[m-kus]

Context

Currently Digest is serialized as two felt252 elements, where first is lower 128 bit and second is higher 128 bit of its numeric representation. It should be noted that Digest -> u256 conversion is little-endian. In particular, for txid (and also block hash, merkle root) the hex string received from RPC/explorer would look the same as u256 integer in hex form. So it would be more natural to serialize the digest in LE as well, to simplify "off-circuit" implementations (e.g. in Python).

Task

• Swap high and low in DigestHash
• Update data generation scripts accordingly
• Regenerate integration tests from utreexo

Depends on #229 (should be merged first)