Issue #1869 (and 4707, 5118, 5473, 1456775): use the new

string <-> float conversion routines to make round(x, n) correctly
rounded for floats x, so that it always agrees with format(x, '.<n>f').

Also fix some other round nuisances, like round(123.456, 1-2**31) giving
an integer rather than a float.