Closes rust-lang#31773

The methods don't return `size` slices, but rather slices of
`size` elements. Sorry!

I sure hope this fix is right

cc @steveklabnik

Closes rust-lang#31773

r? @steveklabnik

fixes rust-lang#31598

r? @alexcrichton

I have made changes and built it after that. Please advise,

rust-lang#31820

…bnik

fixes rust-lang#29332

r? @steveklabnik

r? @steveklabnik
cc @mbrubeck

…labnik

Reference implied that use declarations may appear *only* at the top of blocks and modules, but it is not the case, and the following is valid:

```Rust
fn foo() {
    let x = 92;
    use baz::bar;
}
```

r? @steveklabnik

As a Rust newbie, I found the book's explanation for why the `filter` closure gets a reference very confusing, and tried to figure out why `filter` is somehow less consumptive than `map` -- but it isn't; that's controlled by `iter`/`into_iter`.  I flailed around for a while until @habnabit explained it to me, and in retrospect it is quite obvious :-)