* Save full descriptor instead of only value for private fields.
Decorators can make private fields non-writable, so we need to store this
information somewhere.
The descriptor can also be used to implement private accessors.
* Update member-expression-to-functions
1. Babel using British spellings, so `memoise`
2. Provide a helper `AssignmentMemoiser` class, which will assign the memo'd value with the `n`th access.
* Private properties!
* Fixes
* Tests
* Update helper name
* Fix privates that reference other privates
* Don't extend a builtin
* Rebase
