Rework "breakeven" price as "price-per-uni": ppu

The original implementation of `.calc_be_price()` wasn't correct since
the real so called "price per unit" (ppu), is actually defined by
a recurrence relation (which is why the original state-updated
`.lifo_update()` approach worked well) and requires the previous ppu to
be weighted by the new accumulated position size when considering a new
clear event. The ppu is the price that above or below which the trader
takes a win or loss on transacting one unit of the trading asset and
thus it is the true "break even price" that determines making or losing
money per fill. This patches fixes the implementation to use trailing
windows of the accumulated size and ppu to compute the next ppu value
for any new clear event as well as handle rare cases where the
"direction" changes polarity (eg. long to short in a single order). The
new method is `Position.calc_ppu()` and further details of the relation
can be seen in the doc strings.

This patch also includes a wack-ton of clean ups and removals in an
effort to refine position management api for easier use in new backends:

- drop `updaate_pps_conf()`, `load_pps_from_toml()` and rename
  `load_trands_from_ledger()` -> `load_pps_from_ledger()`.
- extend `PpTable` to have a `.to_toml()` method which returns the
  active set of positions ready to be serialized to the `pps.toml` file
  which is collects from calling,
- `PpTable.dump_active()` which now returns double dicts of the
  open/closed pp object maps.
- make `Position.minimize_clears()` now iterate the clears table in
  chronological order (instead of reverse) and only drop fills prior
  to any zero-size state (the old reversed way can result incorrect
  history-size-retracement in cases where a position is lessened but
  not completely exited).
- drop `Position.add_clear()` and instead just manually add entries
  inside `.update_from_trans()` and also add a `accum_size` and `ppu`
  field to ever entry thus creating a position "history" sequence of
  the ppu and accum size for every position and prepares for being
  and to show "position lifetimes" in the UI.
- move fqsn getting into `Position.to_pretoml()`.