How price syntax works in Ledger and hledger

WIP

In Ledger and hledger

In the journal, a P DATE COMMODITY AMOUNT directive some commodity's market price in some other commodity on DATE. (A timestamp may be added, but is ignored.)
In a posting, AMT @ UNITPRICE declares the per-unit price that was used to convert AMT into the price's commodity. Eg: 2A @ 3B records that 2A was posted, in exchange for 6B.
@@ TOTALPRICE is another form of @, sometimes more convenient. Eg: 2A @@ 5.99B records that 2A was posted in exchange for 5.99B.

In Ledger

Any use of @ also generates an implicit P directive. Eg:
```
2019/1/1
  a  2A @ 3B
  b
```
in the journal is equivalent to writing
```
2019/1/1
  a  2A @ 3B
  b

P 2019/1/1 A 1.5B
```

These need testing:

{UNITPRICE} and {{TOTALPRICE}} are synonyms for @ and @@
{`` UNITPRICE} is like @ but it also causes capital gains transactions to be generated
a @/@@ and a { }/{{ }}/{ `` }/ may be combined; the first is used

In hledger

{ }, {{ }}, and {= } are ignored, and if combined with @/@@, must be written last