GeezerVison

My goal is to explain the underlying concepts first (destructors and then ownership) with plenty of step-by-step examples, so that thoroughly explaining the semantics of mutability, boxes, moves, etc. is a breeze. I'm trying to avoid the comparisons with C++ that were done before, because this should be approachable for people coming from any language. C++ programmers already know these concepts so they aren't the audience that needs to be catered to. Comparisons with other languages can be done in separate documents (like [this one](https://github.com/mozilla/rust/wiki/Rust-for-CXX-programmers)).

This still needs examples for ownership (inherited mutability), owned boxes and managed boxes.

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

Move cognitive_complexity to nursery

As discussed in rust-lang/rust-clippy#5418 (comment); Clippy's current understanding of cognitive complexity is not good enough yet at analyzing code for understandability to have this lint be enabled by default.

changelog: Remove cognitive_complexity from default set of enabled lints

implement semaphores (pthread and macOS APIs)