added the TransactionBuilder interface
added the TransactionBuilder interface

to categorize functions previously in Walle that go towards
building transactions

MrStrong/Sia,ericflo/Sia,NebulousLabs/Sia,mharkus/Sia,mnsl/Sia,ChrisSchinnerl/Sia,ericflo/Sia,kustomzone/Sia,mnsl/Sia,rivine/rivine,mtlynch/Sia,NebulousLabs/Sia,johnathanhowell/Sia,Mingling94/Sia,mtlynch/Sia,ChrisSchinnerl/Sia,mharkus/Sia,MrStrong/Sia,rivine/rivine