d58b7919

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add lot prices

An asset held at a lot price becomes its own commodity: `2 SWDA lot @ 500 EUR`
is an amount of 2 in the commodity "SWDA acquired at 500 EUR per unit", a
distinct key in every balance map from plain `SWDA` and from `SWDA` bought at
another price.

A lot balances at its lot rate, exactly like `@` does, and converts one to one
into the commodity it is a lot of, so it is worth whatever the underlying is
worth rather than what it cost.  Disposal is a negative posting in the same lot,
which leaves the difference between basis and proceeds for a capital gains
posting to absorb.  A lot-free assertion sums across every lot of a symbol, so
an assertion copied off a broker statement stays writable once lots are in play.

This also splits `Currency`, which was doing two jobs: a declared currency, and
whatever a balance is denominated in.  The lot rode on it as a Maybe field,
which is why `declaredCurrency` had to exist.  A `Commodity` is now a currency
or a lot, and one rule decides every site: conversion targets are currencies,
balance keys are commodities.

What follows from the types rather than from a validity rule:

  * a cost converts to a `Currency`, so nothing can convert into a lot
  * a `Lot` holds a `Currency`, so a lot of a lot cannot be built
  * `PostingPrice` has nowhere to put a cost beside a lot

A currency can no longer be called `lot`.

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