Expose functions needed from the voting pool package

We are working on moving the voting pool code into a separate package (see https://github.com/larshesel/btcwallet/tree/move-vp-into-own-package), and to that end we need access to a few resources from the waddrmgr package. Currently we have just added these functions to our own tree to make it work: https://github.com/larshesel/btcwallet/blob/move-vp-into-own-package/waddrmgr/votingpool_bridge.go#L9-L19 and https://github.com/larshesel/btcwallet/blob/move-vp-into-own-package/waddrmgr/votingpool_bridge.go#L28-L32.

* Make the cryptoKey* accessible from outside the Manager to support the voting pool package split.
* Make the newScriptAddress accessible outside the Manager to support the voting pool package split.