iterator: add DoubleEndedIterator concept

This implements the trait for vector iterators, replacing the reverse
iterator types. The methods will stay, for implementing the future
reverse Iterable traits and convenience.

This can also be trivially implemented for circular buffers and other
variants of arrays like strings and `SmallIntMap`/`SmallIntSet`.

The `DoubleEndedIterator` trait will allow for implementing algorithms
like in-place reverse on generic mutable iterators.

The naming (`Range` vs. `Iterator`, `Bidirectional` vs. `DoubleEnded`)
can be bikeshedded in the future.
vec: rm inline(never) hack

just avoid giving an inline hint in the first place
Fix comment indenting properly for Vim files.

Indentation now works correctly on subsequent lines of a multi-line
comment, whether there are leaders (` * `) or not. (Formerly it was
incorrectly doing a two-space indent if there was no leader.)

By default, this no longer puts a ` * ` leader on `/*!` comments, as
that appears to be the current convention in the Rust source code, but
that can easily be re-enabled if desired:

    let g:rust_bang_comment_leader = 1
Fix Vim indentation for inline closures.
auto merge of #7707 : thestinger/rust/double, r=cmr

00da76d r=cmr
6e75f2d r=cmr

This implements the trait for vector iterators, replacing the reverse
iterator types. The methods will stay, for implementing the future
reverse Iterable traits and convenience.

This can also be trivially implemented for circular buffers and other
variants of arrays like strings.

The `DoubleEndedIterator` trait will allow for implementing algorithms
like in-place reverse on generic mutable iterators.

The naming (`Range` vs. `Iterator`, `Bidirectional` vs. `DoubleEnded`)
can be bikeshedded in the future.