[borgar]

Number formats now round in two steps, matching Excel: first the exact value is rounded to 15 significant digits, with a tie going toward zero; then that decimal is rounded to the pattern's last digit, with a tie going away from zero. Digits are always written out in full, so large and small values no longer appear as e+21 or similar.

• 1.005 with 0.00 gives 1.01 (was 1.00)
• 2 ** 70 with #,##0 gives 1,180,591,620,717,410,000,000 (was garbled)
• Engineering notation moves the exponent when rounding carries: 999.96 with ##0.0E+0 gives 1.0E+3
• General rounds a tie at the tenth digit up, as Excel does

This is an alternative to #93, using a faster, arithmetic-based approach.

Notable differences from #93

Engineering notation when rounding carries:

Pattern	Value	#93	This PR
##0.0E+0	999.96	1000.0E+0	1.0E+3
##0.0E+0	999960	1000.0E+3	1.0E+6
##0.0E+0	999999999999999 + 0.5	1000.0E+12	1.0E+15

General with a tie at the tenth digit:

Value	#93	This PR
4.2394791745	4.239479174	4.239479175
19147467.115	19147467.11	19147467.12
42299445.995	42299445.99	42299446

This is because round() now follows the same rules as the formats:

Call	master	This PR
round(1.005, 2)	1	1.01
round(0.285, 2)	0.28	0.29
round(1e15 + 5)	1000000000000005	1000000000000000

Vulgar fractions keep the old rounding, which matches Excel more closely. A known difference is recorded as an expected-failure test.