Review and address feedback about snafu HOT 1 OPEN

From @BurntSushi:

One thing that might help is being super clear in the docs what exactly is generated. I (stupidly, in retrospect) kept trying to call fail on a context selector that had a source field. After thinking about it and more careful reading, I realized that was a nonsensical operation. The generics on the context selector also threw me off, but once I embraced it, I realized it was a nice ergonomic improvement that saved me from having to write some convenience constructors. All this info is in your guide, but it took a few careful read throughs to grok it all.

I haven't finished converting ripgrep yet, so I'll probably have more feedback once it's done, but I've hit two things so far:

1. It seems tricky or impossible to define a recursive error type. If you basically try source: Box<MyError>, then you get an error saying it doesn't impl Borrow. You can add that impl, but context selectors don't really seem to work. (I'm on mobile now, so I can't elaborate on my use case. There are usually alternative designs that don't require a directly recursive error type, but they usually entail making more types.)

2. Sometimes I just want to manually construct an error variant with a source error (e.g., collecting a Vec<MyError>), and doing that is a little weird today since you have to do Context { .. }.into(). Might be nice to have a constructor (like fail) that accepts the source error (unlike fail) on the context selector itself.

from snafu.