Have a pass over metadata validation

[jsdw]

• Loosen struct and enum field ordering now that EncodeAsType and DecodeAsType render the order irrelevant
• Tighten up some other places where we used xor instead of concat_and_hash
• Get rid of sorting and such by relying on XOR to produce order-independent hashes
• merge the per pallet and global metadata hashing logic into one function (We may want to extend this for only specific runtime APIs too once we allow runtime APIs to be stripped from metadata too)

[jsdw]

I merged the per pallet hashing and the global hashing here. The effect is that the per pallet hashing will now also care about any additional details that the global one did (eg runtime APIs).

I think that this is the correct behaviour, but worth a ponder as I might be forgetting something :)

[tadeohepperle]

Looks good to me :)