print fixes, new amount styling api

[simonmichael]

This is a rather extensive set of changes to fix a few issues with print.

Internal changes:

There's a new, consolidated amount styling api, with a new HasAmounts
typeclass. More cleanup is possible.

AmountStyle now makes setting the precision optional.
This simplifies the code for styling amounts, but it complicates
the semantics (Nothing is useful only when setting style).
Not sure if it's the best way.
Also some other fields have been renamed/reordered more mnemonically.

Reports now typically do a final amount styling pass, where they can
tweak amount rendering if needed (and can style only the amounts they
are rendering).

An internal styling pass when converting to cost is no longer needed
and has been dropped. (The initial styling pass in journalFinalise
could perhaps be dropped some day, but for now it is still needed.)

User-visible changes:

print now styles costs in balance assertions consistently.

print no longer does inappropriate rounding in implicit conversions (#2079).

print now shows a disambiguating decimal mark when needed,
for clarity and reparseability.
Eg "1,000" (with , as a thousands separator and no decimal digits) is
now displayed with a decimal mark: "1,000.".
"1 000" (where space is a thousands separator) is less ambiguous,
but we do the same thing (eg "1 000.") for consistency, and also to
help disambiguate when forgetting to quote a numeric commodity symbol
(eg "1234 0" where 1234 is a symbol that should have been in double quotes).

[simonmichael]

Notes on print behaviour:

print shows up to four amounts per posting:
amt - the posting amount
cost - the posting amount's cost
bal - the balance assertion amount
balcost - the balance assertion amount's cost

hledger's amount display styling can be divided into:
basic - everything except precision
prec - precision (number of decimal places)

print's styling behaviour over time:

|         | basic styling                | precision styling            |
| hledger | amt  | cost | bal  | balcost | amt  | cost | bal  | balcost |
| 1.14    | Y    | N    | N    | N       | Y    | N    | N    | N       |
| 1.15    | Y    | N    | N    | N       | Y    | N    | Y    | N       |
| 1.21    | Y    | N    | Y    | N       | Y*   | N    | N    | N       |
| 1.30    | Y    | Y    | Y    | N       | Y*   | N    | N    | N       |
| 1.31    | Y    | Y    | Y    | Y       | N    | N    | N    | N       |

* these add trailing decimal zeros.