Pass txstore.Credit/Debits directly, not pointers.

The Credit and Debits structures are simple wrappers around an
embedded *txstore.TxRecord, as well as an output index in the case of
Credit.  This means that a Credit is at most two words, while a Debits
struct is just one.  To avoid the unnecessary garbage of creating
Credit and Debits structures on the heap (where the underlying
TxRecord likely already is), simply pass around everywhere as
non-pointer types, and modify the receivers for all Credit and Debits
methods to non-pointer receivers since none of them ever modify the
value.