fmt: use the minimum parts array size

fmt: use the minimum parts array size

The comments for flt2dec::to_shortest_str says that we only need a slice
of length 5 for the parts array.  Initializing a 16-part array is just
wasted effort and wasted stack space.  Other functions in the flt2dec
module have similar comments, so we adjust the parts arrays passed to
those functions accordingly.