vim improvements

1. Highlight new lifetime syntax
2. Visually distinguish `unsafe` keyword
3. Add a new file that highlights column 78, to warn when lines get too long
auto merge of #5049 : bstrie/rust/vim, r=catamorphism

1. Highlight new lifetime syntax
2. Visually distinguish `unsafe` keyword
3. Add a new file that highlights column 78, to warn when lines get too long