Problem: implementing custom comparison methods

[yrashk]

While the comparison method to be created would benefit from being able to fall back onto the default implementation, it can't do so as these default methods are private to libfyaml and are not exposed.

Solution: make them available as a public interface

[yrashk]

@pantoniou have you had a chance to look at this, what do you think?

[yrashk]

Actually, I think there's an easier solution: just exposing int fy_token_cmp(struct fy_token *fyt1, struct fy_token *fyt2) in header files. This way, a custom comparator can use it.

I will send a separate PR.

[yrashk]

I sent #87 for this and I've also discovered a bug 🐛 in fy_compare_node_user, see #86