Extract compute pubkey from compute script pubkey

[setavenger]

Description

Split get_silentpayment_script_pubkey into two functions. Adds a distinct function to compute a silentpayment pubkey. Helps other implementations utilising this crate by exposing more low level functions.

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[nymius]

Do you have any other use of this function than generate the pubkeys with both parities? I would prefer to not leave to the user the building of the script pubkey, as it may be a source of mistakes. Maybe reducing the return output of the old function to the two possible script pubkeys is better, although I'm not completely happy with that. Returning both TweakedPublicKeys is another alternative.