Standardize how internal slots are named and linked to. (#1395)
* Standardize how internal slots are named and linked to.
Related to #1112.
This replaces any instance of `[[<dfn>Foo</dfn>]]` with
`<dfn>[[\Foo]]</dfn>`, which means we don't have to worry about
definitions conflicting, and can have a slot whose name matches
an attribute.
Also changed all internal slot names to be camelCase, for consistency.
* Make some names more descriptive.
* Capitalize first character of slot names.
* Fixing a couple internal slot references.
* Fix an instance of [[currentDirection]]