Add Access Handles to spec (#21)

* Add Access Handle to spec, update explainer

* Update explainer examples

* Fix File System typo

* Mark ReadWrite options as optional in explainer

* Replace else with otherwise

* Reply to remaining feedback

* Add right errors to write/create

* Fix read reasult bug

* Remove copy from write

* Update TODOs

* Make read/write options optional

* Update note on write()

* Update write() note

* Clarify WritableStream truncate/cursor interaction

* disambiguate dfn links