MINOR: tools: implement trimming of floating point numbers

MINOR: tools: implement trimming of floating point numbers

When using "%f" to print a float, it automatically gets 6 digits after
the decimal point and there's no way to automatically adjust to the
required ones by dropping trailing zeroes. This function does exactly
this and automatically drops the decimal point if all digits after it
were zeroes. This will make numbers more friendly in stats and makes
outputs shorter (e.g. JSON where everything is just a "number").

The function is designed to be easy to use with snprint() and chunks:

  snprintf:
    flt_trim(buf, 0, snprintf(buf, sizeof(buf), "%f", x));

  chunk_printf:
    out->data = flt_trim(out->area, 0, chunk_printf(out, "%f", x));

  chunk_appendf:
    size_t prev_data = out->data;
    out->data = flt_trim(out->area, prev_data, chunk_appendf(out, "%f", x));