3/4/5-tuple accessors
dbg(ppshow) output improvements

- align single-line output a little better
- start multi-line output on a new line and indent it
dbgAt
show args processing debug output only at level 2+
briefer mixed amount debug output below level 3
more verbose Amount debug output at level 6+
dbgAtM, monadic version of dbgAt
I see, polymorphic let bindings need a proper type sig
force DateSpans to line-wrap better in debug output
less debug output from some low-level functions
an Account now knows how many postings it has
show anumpostings in Account's show instance
test and fix display of zero-balance leaves (fixes #170)
overhaul single- and multi-column balance reports

Changes include:

- flat mode now shows exclusive (subaccount-excluding) balances.
  This is a deviation from ledger, but seems simpler and clearer
  for users and implementors across the various modes.

- in flat mode, --depth now aggregates deeper accounts at the
  depth limit, rather than just excluding them from the report.
  This is more useful.

- in flat mode, --empty no longer shows parent accounts with
  no postings.

- more tests, more debug output, clearer code