Allow self in non-unit struct and enum top-level assert

[octylFractal]

Also allows top-level map to use self instead of this.

Complete with documentation!

[csnover]

This was rebased, and is merged now. I added a couple of additional commits:

1. Tried clarifying the docs and added an example using top-level map
2. Removed the this-compatibility and sanitised the variable
3. Fixed auto-derived assertion failure messages from leaking the internal transformation from self to something else, and added a test for that
4. Fixed nightly lints, which no longer allow message-less panics.