f68771a helpfully enabled the "n" typecode for locale-aware printing of
integers, but not for floats. This commit enables the use of "n" for
locale-aware printing of floats, as in Python, and additionally
introduces the "," format specifier, which can be applied to "d", "f",
and "g" (and their upper-case variants, when present) to force
locale-aware grouping.
"," is particularly useful for floats, as "n" will use exponential
notation for large floats, while ",f" never will.
Unlike in Python, there is no distinction between ",d" and "n" for
integers, or ",g" and "n" for floats. (In Python, "," will always output
a literal comma, while "n" is locale-aware.) In fmtlib, both "," and "n"
are locale-aware.
0a363f2b02