Field offsets

[Rendered view](https://github.com/Zoxc/rfcs/blob/field-offsets/active/0000-field-offsets.md)
My understanding is @Zoxc's primary motivation here is to be able to build a linked list using internal storage instead of the more common external storage. The need for field offsets is so that way a single type can participate in multiple lists simultaneously (by providing multiple node-ptr fields).