Getter substitution is a bit unconventional

[johannhof]

Our getter steps should probably use the form described in https://webidl.spec.whatwg.org/#getter-steps, I.e. start with the right words and use this instead of explicitly declaring handle and types.

Further, steps like Return the invocation of sessionStorage. seem incorrect, there's probably a better way of calling those getters on a different object, maybe just by copy-pasting their steps as some of the caching logic would need to be different anyway, see #19.