Remove types and members marked obsolete in 1.x.

[airbreather]

Throughout the development of 1.x, I've made some mistakes that can only be completely fixed by one or more intentional breaking changes. I intend to follow SemVer 2.0.0, so the most that a 1.x version can do to fix these mistakes is to provide the correct alternatives and mark the incorrect ones as obsolete.

According to SemVer 2.0.0, breaking changes are allowed as long as the major version is incremented, so let's make sure that Cursively 2.0.0 finishes fixing these mistakes.

[airbreather]

I keep briefly forgetting this... the "behavior breaking change" label is because instead of removing the header-aware visitor's default constructor, I'm actually intending to remove the [Obsolete] tag and having it act the same as what it suggests users should do instead.

Technically it's "remove the obsolete default constructor" followed immediately by "add a default constructor that behaves correctly", but since both have to happen at once in order for me to fulfill my goals, a watchful outsider would consider this to be a "behavior breaking change", so I'm counting it.