document implicit lifetime bounds

[tlyu]

Add documentation about implicit lifetime bounds that the compiler
can infer for functions, trait implementations, and structs.
This draws from RFC 2093 (for structs) and RFC 1214 for functions
and trait implementations.

This is built on #1062 and #1060.

It does substantially increase the length of the page it's on, so I would be open to some reorganization.

Fixes #289.

[traviscross]

We talked about this on the lang docs call. This is good work.

There are two things that we want to change before merging this. One is that we'd like the verbiage to state the rules more directly rather than leaning so heavily on the examples. The other is that we'd like to roll this into the section on implied bounds.

[traviscross]

@rustbot author

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tlyu]

I've done a rebase. I'm still refreshing my memory about the subject matter after a few years away, so I'll address the review comments once I've done that.

[tlyu]

It looks like #1261 added a lot of overlapping content. I'm going to see how much overlap there is, and check whether the content that I wrote is still needed.