Add details on how names are introduced.

[ehuss]

This expands on several sections to specify exactly which names are introduced by items. This also contains a few tangential updates, such as explaining what Self is and how it is defined, and a major update to the use chapter.

This does not cover everything; that's a multi-year effort. This is intended as a small step along that journey. One giant gaping hole is covering expansion and peculiarities related to that. The ambiguities section is also incomplete.

Closes #129 cc #487 cc #568

[ehuss]

Thanks so much for the review, I do appreciate it!