* Typings: get rid of "| void" return values

These are not very useful, because the "void" half can never be used.
For `next()` and `prev()` this now declares the actual return value,
`undefined`. For `each()` and `walk*()`, this declares different
overloads for callbacks that do or do not return booleans.

* Typings: Node.raw() returns string | undefined

* Container.every() and .some() don't take thisArg