Add utility methods for abstracting Payment and Asset Transfer transactions

[barnjamin]

There is often a need to work with both Algo payments and Asset transfers but the code duplication can be a pain.

Implement some set of methods to abstract the differences between them to allow a nicer API

[algoanne]

Situations where it would be useful some have some generic abstraction:

• in a smart contract, when constructing an inner transaction.
• in a smart contract, when referring to other transactions in the group.
• when constructing a (top-level) transaction group that includes an app call and needs to include a pay or axfer