Rollup merge of #40514 - stjepang:inline-then-ordering, r=alexcrichton
Rollup merge of #40514 - stjepang:inline-then-ordering, r=alexcrichton

Inline functions Ordering::{then, then_with}

@jongiddy noticed bad performance due to the lack of inlining on `then`
and `then_with`. I confirmed that inlining really is the culprit by
creating a custom `then` function and repeating his benchmark on my
machine with and without the `#[inline]` attribute.

The numbers were exactly the same on my machine without the attribute.
With `#[inline]` I got the same performance as I did with manually
inlined implementation.

The problem was reported in #37053.

GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust