MINOR: tools: add a float-to-ascii conversion function

MINOR: tools: add a float-to-ascii conversion function

We already had ultoa_r() and friends but nothing to emit inline floats.
This is now done with ftoa_r() and F2A/F2H. Note that the latter both use
the itoa_str[] as temporary storage and that the HTML format currently is
the exact same as the ASCII one. The trailing zeroes are always timmed so
these outputs are usable in user-visible output.