fix(BRIDGE-610): structurally compare literals if byte-for-byte comparison fails

[annatar]

Add a structural equivalence check before deciding on whether to update or create a new literal.

[ElectroNafta]

Very nicely done!

A couple of things to note:

• I think we filter out display names for the message hash. So changes in the display names won't be taken into account. Not a real issue, and extremely hard to hit. But perhaps worth opening up a ticket just so we know of the debt. We may need to tweak it further down.
• We don't tolerate MIME part re-ordering, which may happen in some cases. This was already part of the code for a long time so it shouldn't pose an issue, but something to be aware of.