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.

r? @nikomatsakis

Downgrade inefficient_to_string to pedantic

From the [documentation](https://rust-lang.github.io/rust-clippy/master/index.html#inefficient_to_string):

> ```diff
> - ["foo", "bar"].iter().map(|s| s.to_string());
>
> + ["foo", "bar"].iter().map(|&s| s.to_string());
> ```

I feel like saving 10 nanoseconds from the formatting machinery isn't worth asking the programmer to insert extra `&` / `*` noise in the *vast* majority of cases. This is a pedantic lint.

changelog: Remove inefficient_to_string from default set of enabled lints

Rollup of 11 pull requests

Successful merges:

 - rust-lang#5406 (Fix update_lints)
 - rust-lang#5409 (Downgrade let_unit_value to pedantic)
 - rust-lang#5410 (Downgrade trivially_copy_pass_by_ref to pedantic)
 - rust-lang#5412 (Downgrade inefficient_to_string to pedantic)
 - rust-lang#5415 (Add new lint for `Result<T, E>.map_or(None, Some(T))`)
 - rust-lang#5417 (Update doc links and mentioned names in docs)
 - rust-lang#5419 (Downgrade unreadable_literal to pedantic)
 - rust-lang#5420 (Downgrade new_ret_no_self to pedantic)
 - rust-lang#5422 (CONTRIBUTING.md: fix broken triage link)
 - rust-lang#5424 (Incorrect suspicious_op_assign_impl)
 - rust-lang#5425 (Ehance opt_as_ref_deref lint.)

Failed merges:

 - rust-lang#5345 (Add lint for float in array comparison)
 - rust-lang#5411 (Downgrade implicit_hasher to pedantic)
 - rust-lang#5428 (Move cognitive_complexity to nursery)

r? @ghost

changelog: rollup