Replace to_string compare with == object compare

[mgreter]

This a follow up for sass/sass#2229. This breaks a spec test which should hopefully be the same when sass/sass#2232 is released. There might be more cases where we use to_string to compare objects. This could lead to some regressions, as I think the compare functions do lack some edge cases. But we want them to surface, as we already use them in our hash map implementation. It is also faster than converting to a string before (specially if we can determine inequality early).

[mgreter]

This has some unexpected failures in https://ci.appveyor.com/project/sass/libsass/build/1.0.3068/job/ih3by0c12b0fc0jv! Needs further investigations ...

[mgreter]

Seems this is a bit too early. The failing spec test is is-superselector(':-moz-any(.foo, .bar)', ':-s-any(.foo, .bar)'); which crashes with a segfault, seems to indicate that we store some kind of selector inside those wrapped selectors. The compare operator doesn't seem to like it. Might be that we have some crude static cast lingering somewhere. The solution should be the same as in #2300

[mgreter]

It seems I was able to fix the issue. Not it is failing only the expected spec test.

[mgreter]

Up on further review this needs some more changes as it is not equivalent to ruby sass now. There seems to be some path in unify_with that does not properly use the == operator.