RFC: Finalizing more naming conventions

This conventions RFC tweaks and finalizes a few long-running de facto
conventions, including capitalization/underscores, and the role of the `unwrap` method.

See [this RFC](https://github.com/rust-lang/rfcs/pull/328) for a competing proposal for `unwrap`.

[Rendered](https://github.com/aturon/rfcs/blob/finalizing-naming-conventions/text/0000-finalizing-naming-conventions.md)
Note that there are precedents in both directions here, and I'm not sure it's clear that we should draw a distinction between these two kinds of variables.

Looking around, we've got:

* All caps: [Python](http://legacy.python.org/dev/peps/pep-0008/#constants), [Ruby](https://github.com/bbatsov/ruby-style-guide#screaming-snake-case), [Java](http://www.oracle.com/technetwork/java/codeconventions-135099.html), [Javascript](http://javascript.crockford.com/code.html)
* Camel case: [C++](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Constant_Names) (kinda), [Go](http://stackoverflow.com/questions/22688906/go-naming-conventions-for-const) (I think),

I'll also point out that "SCREAMING_SNAKE_CASE" is abrasive probably because it has the word "screaming" in all caps. Something like `MAX_WIDTH` vs `MaxWidth` seems less abrasive to me