Fix errors in HCursor

[travisbrown]

The important part here is first and last in HCursor—the current implementations have copy-paste errors that mean that these operations are broken on both HCursor and ACursor.

I've also corrected a couple of similar cases of copy-paste errors in history tracking, and added two missing CursorOpElement types. I've made up what I think are reasonable string representations for these, but I'm happy to change them.

I haven't added any tests, since HCursor isn't currently tested directly. I'd be happy to add something for this in a separate PR.

[seanparsons]

That's a good spot, they must've been wrong for ages.

[seanparsons]

Looks like a massive blind spot in our tests.

[travisbrown]

@seanparsons I'm still fumbling around with testing ideas in circe, but I think abstracting over Cursor, HCursor, and ACursor is a nice way to make this specific area easier—you just write a bunch of test cases and then reuse them for each cursor type.

[seanparsons]

Might just steal that.