Slight performance improvement in keysort/3 and ukeysort/3.
Fixed bug in to_float/1.

The to_float/1 function would return an excpetion for dot initiated floats
rather than an error, e.g., 'bstring:to_float(<<".0">>).' would result in
** exception error: bad argument
     in function  list_to_float/1
        called as list_to_float(".0")
     in call from bstring:to_float_frac/2 (src/bstring.erl, line 596).