Rollup merge of #52942 - llogiq:smallvec-opt, r=Mark-Simulacrum

Rollup merge of #52942 - llogiq:smallvec-opt, r=Mark-Simulacrum

Another SmallVec.extend optimization

This improves SmallVec.extend even more over #52859 while making the code easier to read.

Before

```
test small_vec::tests::fill_small_vec_1_10_with_cap  ... bench:          31 ns/iter (+/- 5)
test small_vec::tests::fill_small_vec_1_10_wo_cap    ... bench:          70 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_1_50_with_cap  ... bench:          36 ns/iter (+/- 3)
test small_vec::tests::fill_small_vec_1_50_wo_cap    ... bench:         256 ns/iter (+/- 17)
test small_vec::tests::fill_small_vec_32_10_with_cap ... bench:          31 ns/iter (+/- 5)
test small_vec::tests::fill_small_vec_32_10_wo_cap   ... bench:          26 ns/iter (+/- 1)
test small_vec::tests::fill_small_vec_32_50_with_cap ... bench:          49 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_32_50_wo_cap   ... bench:         219 ns/iter (+/- 11)
test small_vec::tests::fill_small_vec_8_10_with_cap  ... bench:          32 ns/iter (+/- 2)
test small_vec::tests::fill_small_vec_8_10_wo_cap    ... bench:          61 ns/iter (+/- 12)
test small_vec::tests::fill_small_vec_8_50_with_cap  ... bench:          37 ns/iter (+/- 3)
test small_vec::tests::fill_small_vec_8_50_wo_cap    ... bench:         210 ns/iter (+/- 10)
```

After:

```
test small_vec::tests::fill_small_vec_1_10_wo_cap    ... bench:          31 ns/iter (+/- 3)
test small_vec::tests::fill_small_vec_1_50_with_cap  ... bench:          39 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_1_50_wo_cap    ... bench:          35 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_32_10_with_cap ... bench:          37 ns/iter (+/- 3)
test small_vec::tests::fill_small_vec_32_10_wo_cap   ... bench:          32 ns/iter (+/- 2)
test small_vec::tests::fill_small_vec_32_50_with_cap ... bench:          52 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_32_50_wo_cap   ... bench:          46 ns/iter (+/- 0)
test small_vec::tests::fill_small_vec_8_10_with_cap  ... bench:          35 ns/iter (+/- 4)
test small_vec::tests::fill_small_vec_8_10_wo_cap    ... bench:          31 ns/iter (+/- 0)
test small_vec::tests::fill_small_vec_8_50_with_cap  ... bench:          40 ns/iter (+/- 15)
test small_vec::tests::fill_small_vec_8_50_wo_cap    ... bench:          36 ns/iter (+/- 2)
```