This refactors some of the `ledger` code so that it's convenient
to write short tests that operate on a fairly complete ledger,
including rewards payouts.  Previously, tests used genesis(), but
genesis() build the genesis block internally "by hand" rather
than using MakeGenesisBlock, so it missed some details (like
setting up RewardsState). Presumably, this was because
MakeGenesisBlock was in the `data` package, and could not be
imported.  That is the motivation behind moving it, and some
related code, to bookkeeping (where various Genesis related code
already existed).

The txntest packaged is motivated purely by a desire for more
concise tests.  It allows for the construction of
transaction.Transaction objects concisely, and we can add all
sort of conveneince routines here that would not make sense in
the production code (turning these into SignedTxns,
SignedTxnWithADs, TransactionGroups, etc).