vim: assert and pure keywords were removed
vim: highlight ref + static as storage specifiers

lifetimes and globals are now the only two places static is used, and
'static isn't matched by this
vim: mark Todo as contained and rm unsafe from it

It's nice to make unsafe stand out, but this way isn't correct because it
highlights it in comments.
vim: separate the conditional keywords
vim: use Operator group for 'as'
auto merge of #5636 : thestinger/rust/vim, r=luqmana