cargo: bump rust-simplicity version

[KyrylR]

Since 0.8.0, there are breaking public changes (quick AI summary):

• Final::two_two_n now returns Result instead of its previous behavior.
• Policy::Assembly / the Assembly fragment was removed.
• Public Elements/hash types move from Elements 0.25 and bitcoin_hashes 0.14 to Elements 0.27 and bitcoin_hashes 1.2.

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Therefore I would go for the minor version bump

[stringhandler]

I think @apoelstra is still in the middle of adding canonical CMRs so might be better to wait

[apoelstra]

Yeah, I'd like to sneak #377 in there at least. I spoke to Kyrylo out of band and he said he could wait a bit. (But if not, it's no big deal to just do two releasese.)