Highlight sigils and operators in Vim.

Sigil highlighting isn't perfect (especially how it handles ``&``) but
after having used it for a week I feel it to be considerably nicer than
nothing. As usual, if you don't like it, you can turn it off easily by
overriding the default highlighting.

Generics are not handled specially; this means that for something like
``S<T>``, the ``<`` and ``>`` are highlighted as operators. For myself,
I like this, and there is no way to make it properly context aware
without expanding the syntax matching enormously.

Also, special characters are highlighted properly in strings/chars, e.g.
``"\x00"`` or ``'\Ufedcba98'`` appropriately.
Vim highlighting: remove `foreach` keyword
auto merge of #8251 : chris-morgan/rust/vim-sigil-highlighting, r=huonw

Sigil highlighting isn't perfect (especially how it handles ``&``) but
after having used it for a week I feel it to be considerably nicer than
nothing. As usual, if you don't like it, you can turn it off easily by
overriding the default highlighting.

Generics are not handled specially; this means that for something like
``S<T>``, the ``<`` and ``>`` are highlighted as operators. For myself,
I like this, and there is no way to make it properly context aware
without expanding the syntax matching enormously.

Also, special characters are highlighted properly in strings/chars, e.g.
``"\x00"`` or ``'\Ufedcba98'`` appropriately.