(PUP-2663) Allow string to contain leading sign in numeric conversion

This makes it possible to use an unary +/- in a string. The sign
of the converted value abides by the unary string sign. White space is
allowed around the sign, and at the end of the string.

'+ 2' + '- 2' == 0

This is of value when dealing with original data in string form
that needs to be converted. Without this it is especially difficult to
pass a negative value around in string form.