Safer constructor writing for self-referential types

[danakj]

Safer constructor writing for self-referential types

[mcy]

Hey Dana,

I got really hosed and never got back to this 🙃.

I just pushed some commits that add New::with(), which can be used for post-construction work. For example:

new::of(blah).with(|this| /* this: Pin<&mut Self> */)

The new docs for the front matter of the crate demonstrate this pattern on Unmoveable. I need to page in most of this design work at some point, but this should take care of almost all of the easy cases.

(Oh, and FYI: all of the names in the crate have changed to reflect what I used in the RustConf talk, since I felt those were more succinct.)