iterator_base::value_type should not be const

[horenmar]

I had an issue open against my own project, that boils down to yaml-cpp providing iterators, whose value_type is const. This should not happen, values are immutable by definition*, and it causes issues in consuming code that doesn't expect this.

From a quick glance, I think it is fixable just by adding std::remove_const_t here.

* this gets a bit muddy with reference-like types, but there it is generally expected that it is the reference that cannot be modified, not the referenced thing.