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

[petrochenkov]

"Constructor" typically refers only to the value namespace component.
The type namespace component is a "variant itself", or a "variant type" when rust-lang/rfcs#2593 gets implemented.
(It's unfortunate that that RFC is postponed because it cannot decide how to construct values of variant types better, which is an entirely orthogonal question to introducing variant types themselves, at least in theory.)

[ehuss]

Can you clarify something here? The tuple-struct constructor in the value namespace doesn't currently have much of a purpose, does it? At least, I can't think of a way it could be used. The only thing I noticed is that you are not allowed to shadow it, as in:

enum E {
    V1 {f1: i32}
}

fn foo() {
    use E::*;
    let V1 = 123; // ERROR: let bindings cannot shadow struct variants
}

[petrochenkov]

The value namespace component for braced variants was removed in rust-lang/rust#103578.

[ehuss]

Thanks!

[ehuss]

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

[ehuss]

@traviscross I have rebased, and pushed a small fix for this. I'm not certain what else has changed or is missing since it was written, though. I though I had some additional notes somewhere, and if I find them I'll follow up later.

[petrochenkov]

LGTM except for the braced variant constructor detail - #1052 (comment).