Rollup merge of #48296 - ishitatsuyuki:exp-unblow, r=nikomatsakis

Fix exponential projection complexity on nested types

This implements solution 1 from https://github.com/rust-lang/rust/issues/38528#issuecomment-366263076.

The code quality is currently extremely poor, but we can improve them during review.

Blocking issues:

- we probably don't want a quadratic deduplication for obligations.
- is there an alternative to deduplication?

Based on #48315.

Needs changelog. Noticable improvement on compile time is expected.

Fix #38528
Close #39684
Close #43757

diff --cc src/librustc/traits/mod.rs
Simple merge

diff --cc src/librustc/traits/project.rs
Simple merge

diff --cc src/librustc/traits/select.rs
Simple merge