Frozen2 serialized representation not portable across different byte orders

[mhx]

I'm using Frozen2 for metadata storage in a read-only filesystem. In the process of porting the code to big-endian architectures, I realized that the metadata produced on little-endian systems wasn't readable. In fact, Frozen2 cannot currently be used at all on big-endian systems: Frozen2 cannot correctly read back its own serialized representation produced on big-endian.

The root cause is that folly::Bits, which is the backbone of Frozen2, does not work correctly on big-endian. I don't think folly::Bits was designed to be portable, as the code explicitly mentions that the storage is expected to be little-endian.

Fortunately, it is relatively straightforward to fix folly::Bits by enforcing the assumption of little-endian storage using folly::Endian. There's a pull request that does just that: facebook/folly#2484. The added code is a no-op on little-endian systems, so it shouldn't introduce a performance regression.

With that change, Frozen2 works fine on big-endian systems and the serialized representation can be freely exchanged between different byte orders.