Constructing a pubkey using `PublicKey::from_slice` can fail for reasons
other than just incorrect length - we should not be using `expect` but
rather returning the error.
A purist might argue that we are now returning a nested error type with
an unreachable variant:
`ParsePublicKeyError::Encoding(FromSliceError::InvalidLength)`
Is this acceptable or do we want to further improve this?
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