Auto merge of #52738 - ljedrz:push_to_extend, r=eddyb

Replace push loops with extend() where possible

Or set the vector capacity where I couldn't do it.

According to my [simple benchmark](https://gist.github.com/ljedrz/568e97621b749849684c1da71c27dceb) `extend`ing a vector can be over **10 times** faster than `push`ing to it in a loop:

10 elements (6.1 times faster):
```
test bench_extension ... bench:          75 ns/iter (+/- 23)
test bench_push_loop ... bench:         458 ns/iter (+/- 142)
```

100 elements (11.12 times faster):
```
test bench_extension ... bench:          87 ns/iter (+/- 26)
test bench_push_loop ... bench:         968 ns/iter (+/- 3,528)
```

1000 elements (11.04 times faster):
```
test bench_extension ... bench:         311 ns/iter (+/- 9)
test bench_push_loop ... bench:       3,436 ns/iter (+/- 233)
```

Seems like a good idea to use `extend` as much as possible.

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