Tests for Int64::Add(), Int64::Sub()

Also added support for numeric and, or, xor operations, and made a very
minor tweak to Add(), Sub(): we now allocate value after checking for
arguments, since we don't need it if the argument is missing.
Permit number values larger than int32

JavaScript uses double-precision floating point, and can represent
integer numbers up to roughly Math.pow(2, 53) - this tests for Number
arguments outside the int32 range.