… C++'s std::nth_element (a.k.a. quickselect)

Add some more notes to the documentation:

- Mention that the median can be found if we used `len() / 2`.
- Mention that this function is usually called "kth element" in other libraries.

Address some comments in PR:

- Change wording on some of the documentation
- Change recursive function into a loop

Update name to `partition_at_index` and add convenience return values.

Address reviewer comments:

- Don't swap on each iteration when searching for min/max element.
- Add some docs about when we panic.
- Test that the sum of the lengths of the output matches the length of the input.
- Style fix for for-loop.

Address more reviewer comments

Fix Rng stuff for test

Fix doc test build

Don't run the partition_at_index test on wasm targets

Miri does not support entropy for test partition_at_index

The `InlineAsm` variant is extremely rare, and `mir::Statement` often
contributes significantly to peak memory usage.

Add 'partition_at_index/_by/_by_key' for slices.

This is an analog to C++'s std::nth_element (a.k.a. quickselect).

Corresponds to tracking bug rust-lang#55300.

…ited_again, r=KodrAus

improve worst-case performance of BTreeSet intersection v3

Variation of [rust-lang#59078](rust-lang#59078) with `Intersection` remaining a struct

r? @scottmcm

…ion-elem, r=eddyb

Remove adt_def from projections and downcasts in MIR

As part of optimizing generator layouts in MIR, we'd like to allow downcasting generators to variants which do not have a corresponding `def_id`, since they are created by the compiler.

This refactor hopes to allow that, without regressing perf.

r? @eddyb

…=pnkfelix

Shrink `mir::Statement`.

The `InlineAsm` variant is extremely rare, and `mir::Statement` often
contributes significantly to peak memory usage.