Refactored sudoku benchmark to use traits and added some tests
auto merge of #5420 : boggle/rust/sudoku-with-traits, r=graydon

Came back to look at rust and found out that this had been broken by some refactoring work. Fixed and now has tests.
Update rust.vim

Column limit 78 -> 100.
librustc: Convert all uses of old lifetime notation to new lifetime notation. rs=delifetiming
libsyntax: Stop parsing old lifetimes, except for the ones on data type declarations.
librustc: Make the compiler ignore purity.

For bootstrapping purposes, this commit does not remove all uses of
the keyword "pure" -- doing so would cause the compiler to no longer
bootstrap due to some syntax extensions ("deriving" in particular).
Instead, it makes the compiler ignore "pure". Post-snapshot, we can
remove "pure" from the language.

There are quite a few (~100) borrow check errors that were essentially
all the result of mutable fields or partial borrows of `@mut`. Per
discussions with Niko I think we want to allow partial borrows of
`@mut` but detect obvious footguns. We should also improve the error
message when `@mut` is erroneously reborrowed.