Print a Float in the bill text with two decimals instead of a whole number

[Thayorns]

Since the fix for #1026, Fbe::Award::Bill#line rounded every Float in ${...} to a whole number. That removed the floating point noise, but it also turned a rate of 0.05 into 0, so the bill said "at 0 points each" while the bylaw of the same policy said 0.05, as #1219 shows.

Now a Float in the text of a bill line is rounded to two decimals, and the fraction is dropped when it is zero. The case from #1026 still reads "2 days", a rate reads "0.05", and a tiny negative value reads "0", not "-0.0". The points themselves are still rounded by whole, as before.

One visible change: the resolved-bug-was-rewarded bylaw now tells a contributor "12.5 days of delay" where it said "13 days" for 300 hours.

The new tests cover a random rate, a negative rate, a third rounded to 0.33, a whole Float printed without a fraction, and a tiny negative Float printed as zero.

Closes #1219

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.