doc: Add "A 30-minute Introduction to Rust"

By Steve Klabnik.
doc: Edit intro
Only check --crate-type flags if present.

Before, normal compilation and the --crate-file-name flag would
generate output based on both #![crate_type] attributes and
--crate-type flags. Now, if one or more flag is specified by command
line, only those will be used.

Closes #11573.
doc: Address feedback about intro
auto merge of #13416 : brson/rust/30min, r=alexcrichton

This is intended to be the first thing somebody new to the language reads about Rust. It is supposed to be simple and intriguing, to give the user an idea of whether Rust is appropriate for them, and to hint that there's a lot of cool stuff to learn if they just keep diving deeper.

I'm particularly happy with the sequence of concurrency examples.
auto merge of #13489 : JustAPerson/rust/crate-file-name, r=alexcrichton

Before, the `--crate-file-name` flag only checked crate attributes for
possible crate types. Now, if any type is specified by one or more
`--crate-type` flags, only the filenames for those types will be
emitted, and any types specified by crate attributes will be ignored.
Support lifetime suggestion for method

This includes a change to the way lifetime names are generated. Say we
figure that `[#0, 'a, 'b]` have to be the same lifetimes, then instead
of just generating a new lifetime `'c` like before to replace them, we
would reuse `'a`. This is done so that when the lifetime name comes
from an impl, we don't give something that's completely off, and we
don't have to do much work to figure out where the name came from. For
example, for the following code snippet:

```rust
struct Baz<'x> {
    bar: &'x int
}

impl<'x> Baz<'x> {
    fn baz1(&self) -> &int {
        self.bar
    }
}
```

`[#1, 'x]` (where `#1` is BrAnon(1) and refers to lifetime of `&int`)
have to be marked the same lifetime. With the old method, we would
generate a new lifetime `'a` and suggest `fn baz1(&self) -> &'a int`
or `fn baz1<'a>(&self) -> &'a int`, both of which are wrong.
auto merge of #13164 : ktt3ja/rust/lifetime-suggestion-method, r=nmatsakis

This includes a change to the way lifetime names are generated. Say we
figure that `[#0, 'a, 'b]` have to be the same lifetimes, then instead
of just generating a new lifetime `'c` like before to replace them, we
would reuse `'a`. This is done so that when the lifetime name comes
from an impl, we don't give something that's completely off, and we
don't have to do much work to figure out where the name came from. For
example, for the following code snippet:

```rust
struct Baz<'x> {
    bar: &'x int
}

impl<'x> Baz<'x> {
    fn baz1(&self) -> &int {
        self.bar
    }
}
```

`[#1, 'x]` (where `#1` is BrAnon(1) and refers to lifetime of `&int`)
have to be marked the same lifetime. With the old method, we would
generate a new lifetime `'a` and suggest `fn baz1(&self) -> &'a int`
or `fn baz1<'a>(&self) -> &'a int`, both of which are wrong.
rustc: Make std_inject valid for pprust
pprust: Handle multi-stmt/no-expr `ExprFnBlock`

Fixes #12685