Sprout is needed because pow, log2 and log10 are constexpr on gcc,
but that's nonstandard. Sprout provides the constexpr version of
those functions.
Also fix warnings. I still get plenty of warnings about some suggested
paretheses, but it seems to be a bug from clang. See
https://llvm.org/bugs/show_bug.cgi?id=21629 for the bug report.
This also fixes a subtle bug: the size passed to MaxSizedArray in
int_to_string() was calculated on the max value of type F, but when
F was a signed type the max would only be for example 31 bits.
Conversion of negative numbers would then fail with an assertion.
