Provide a more helpful error for tests that fail due to noexec

The rustdoc tests create and execute a file in a temporary directory. By
default on UNIX-like platforms this is in `/tmp`, which some users mount
with the `noexec` option. In those cases, the tests fail in a mysterious
way. This change adds a note that suggests what the problem might be, if
the error looks like it could have been caused by the `noexec` setup.

Closes #12558
rustc: Remove codemap and reachable from metadata encoder
Fix syntax::ext::deriving{,::*} docs formatting.

The most significant fix is for `syntax::ext::deriving::encodable`,
where one of the blocks of code, auspiciously containing `<S>` (recall
that Markdown allows arbitrary HTML to be contained inside it), was not
formatted as a code block, with a fun but messy effect.
Highlight the `mod` in `extern mod x;` as Error.

Just like the bare keyword `crate` is highlighted as Error (a little
dubious, actually, given macros), `mod` is invalid after `extern`: it's
obsolete syntax.
Downgrade `do` to a reserved keyword in Vim.

This means it gets highlighted as Error by default.
Update prelude items in Vim syntax.
Fix Vim section movements for standard Rust style.

(Expressed another way: make `[[` et al. work with the curly brace at
the end of a line as is standard Rust style, not just at the start is it
is by default in Vim, from K&R style.)

This came out of #11492, where a simpler but less effective technique
was initially proposed; some discussion of the techniques, ways and
means can be found there.

There are still a few caveats:

- Operator-pending mode behaves differently to the standard behaviour:
  if inside curly braces, it should delete up to and including the
  closing of the outermost curly brace (that doesn't seem to me
  consistent with documented behaviour, but it's what it does). Actual
  behaviour (the more logical and consistent, in my opinion): up to the
  start of the next outermost curly brace.

- With folding enabled (`set fdm=syntax`), `[[` and `]]` do not behave
  as they should: the default behaviour treats an entire closed fold as
  one line for these purposes while this code does not (I explicitly
  `set nofoldenable` in the function—the side-effects are worse with
  folds enabled), leading to unexpected behaviour, the worst of which is
  `[[` and/or `]]` not working in visual mode on a closed fold (visual
  mode keeps it at the extreme end of the region line of the folded
  region, so it's always going back to the opening line of that fold and
  immediately being shoved back to the end by visual mode).

- `[[` and `]]` are operating inside comments, whereas the standard
  behaviour skips comments.

- The viewport position is sometimes changed when it should not be
  necessary.
Update Vim syntax file last change date.
path: clean up some lint warnings and an obsolete comment

Get rid of the unnecessary parenthesies that crept into some macros.
Remove a FIXME that was already fixed.
Fix a comment that wasn't rendering correctly in rustdoc.
path: Implement windows::make_non_verbatim()

make_non_verbatim() takes a WindowsPath and returns a new one that does
not use the \\?\ verbatim prefix, if possible.
Improve the ctags function regular expression.

Before it would only catch lines starting `fn` or `pub fn`.

Now it can cope with:

- attributes (e.g. `#[test] fn`)
- external functions (e.g. `extern fn`, `extern "C" fn`)
- unsafe functions (e.g. `unsafe fn`)

… and any correct combination of these
(e.g. `#[test] extern "C" unsafe fn`).
Documentation : Tutorial improvement...

Refactoring examples on implementation of generics for linked list.
Fixing typo of 'Note's for coherancy.

Adding internal links inside the tutorial example with traits,
generics etc...
Minor modifications to Axel's tutorial improvements (see also #12472).
std: Export the select! macro

Mark it as #[experimental] for now. In theory this attribute will be read in the
future. I believe that the implementation is solid enough for general use,
although I would not be surprised if there were bugs in it still. I think that
it's at the point now where public usage of it will start to uncover hopefully
the last few remaining bugs.

Closes #12044
Fix typo in doc of Binary trait in std::fmt
Fix bytepos_to_file_charpos.

Make bytepos_to_charpos relative to the start of the filemap rather than its previous behaviour which was to be realtive to the start of the codemap, but ignoring multi-byte chars in earlier filemaps. Rename to bytepos_to_file_charpos. Add tests for multi-byte chars.