cpp: construct bindings that take a default context

[tobiasgrosser]

Here an idea how #14 could be addressed.

Not sure what to do with constructors that do not take any other arguments. How should the be distinguished from a default constructor that constructs an empty object. This currently happens only with isl_ast_build.

[tobiasgrosser]

Here how the bindings would look like:

void test_default_ctx() {
	isl::set S1("{ [1] } ");
	isl::set S2("{ [2] } ");

	isl::set S12("{ [1] ; [2] } ");

	assert(S1.unite(S2).is_equal(S12));
}