Narrower return types than `P2Ret`

[kyranjamie]

Currently P2Ret acts as a catch-all for the payment methods. It'd be more developer/typescript friendly to avoid the optional props, and return narrower payment-specific types.

For example, if I call the p2sh method, the compiler should know that redeemScript is defined. Usage of the library would be cleaner, in that fewer potentially undefined checks are needed.

(and thanks for all the great work Paul, this lib is a godsend)