Named and parametric lifetimes

[nrc]

Should also cover lifetime elision. I prefer to cover lifetime params in depth before getting on to the elision rules.

Not sure about the ordering with this tut. On one hand I want it to come pretty soon after borrowed references. OTOH, covering some more of the basics first might actually be more useful, but then we get into tricky error messages and elision issues.

[nrc]

Remember `'static'. And subtype variance.

[nrc]

And lifetime bounds on both lifetimes and traits (cc #2).