Revised version of Fred's patch, including support for ~ operator.

If the unary +, -, or ~ operator is applied to a constant, don't
generate a UNARY_xxx opcode. Just store the approriate value as a
constant.  If the value is negative, extend the string containing the
constant and insert a negative in the 0th position.

For ~, compute the inverse of int and longs and use them directly, but
be prepared to generate code for all other possibilities (invalid
numbers, floats, complex).