Modernise some Vim syntax highlighting.

- Remove highlighting of ``L"..."`` (obsolete syntax)
- Remove backslash at end of line being a line continuation always
  (obsolete syntax; this only affects comments, actually)
- Add highlighting for backslash at end of line and leading whitespace
  on the following line inside a string (a genuine line continuation)
Update a handful of keywords highlighted in Vim.

I added a few and removed a few and corrected a couple, all with
reference to the prelude. It ends up a slightly arbitrary decision
precisely what ends up in and what doesn't, unfortunately.
Highlight everything in the prelude in Vim.

This is a rather more extensive change than the last, but *ever* so much
easier to maintain reasonably, as there's then something to track
directly.
Update highlighting for prelude changes.
Silence fo+=j error for users of Vim < 7.3.541.
auto merge of #8951 : chris-morgan/rust/vim-update-highlighting, r=alexcrichton

Several changes with appropriate commit messages to explain them.

The final two commits, highlighting everything in the prelude, may be a little controversial. I think it's the sensible way forward with it.