Auto merge of #36734 - nnethercote:fix-HashSet-sizing, r=Aatch

Auto merge of #36734 - nnethercote:fix-HashSet-sizing, r=Aatch

Don't allocate during default HashSet creation.

The following `HashMap` creation functions don't allocate heap storage for elements.
```
HashMap::new()
HashMap::default()
HashMap::with_hasher()
```
This is good, because it's surprisingly common to create a HashMap and never
use it. So that case should be cheap.

However, `HashSet` does not have the same behaviour. The corresponding creation
functions *do* allocate heap storage for the default number of non-zero
elements (which is 32 slots for 29 elements).
```
HashMap::new()
HashMap::default()
HashMap::with_hasher()
```
This commit gives `HashSet` the same behaviour as `HashMap`, by simply calling
the corresponding `HashMap` functions (something `HashSet` already does for
`with_capacity` and `with_capacity_and_hasher`). It also reformats one existing
`HashSet` construction to use a consistent single-line format.

This speeds up rustc itself by 1.01--1.04x on most of the non-tiny
rustc-benchmarks.