Expose a transaction store and related functions.

Several of the functions require a map of contextual transaction data to
use as a source for referenced transactions.  This commit exports the
underlying TxData type and creates a new type TxStore, which is a map of
points to the under TxData.  In addition, this commit exposes a new
function, FetchTransactionStore, which returns a transaction store
(TxStore) containing all of the transactions referenced by the passed
transaction, as well as the existing transaction if it already exists.

This paves the way for subsequent commits which will expose some of the
functions which depend on this transaction store.