txscript: Optimize ExtractPkScriptAddrs stakegen.
txscript: Optimize ExtractPkScriptAddrs stakegen.

This continues the process of converting the ExtractPkScriptAddrs
function to use the optimized extraction functions recently introduced
as part of the typeOfScript conversion.

In particular, this converts the detection for stake-generation-tagged
pay-to-pubkey-hash and pay-to-script-hash scripts.

jrick/dcrd,decred/dcrd,jrick/dcrd,decred/dcrd