Add RowSelection::iter(), Into<Vec<RowSelector>> and example

[alamb]

Which issue does this PR close?

Closes #3172

Rationale for this change

see #3172 (Datafusion code wants to count the number of selected items)

What changes are included in this PR?

1. Add RowSelection::iter()
2. Add Into<Vec<RowSelector>> (there was only into VecDeque for some reason)
3. Add a Doc example

Are there any user-facing changes?

new functions and docs

[tustvold]

Suggested change

	pub fn iter(&self) -> impl Iterator<Item = &RowSelector> {
	self.selectors.iter()
	pub fn iter(&self) -> impl Iterator<Item = RowSelector> {
	self.selectors.iter().copied()

[alamb]

Is there any reason to copy the RowSelectors around?

[tustvold]

Not especially, was just an idle suggestion

[ursabot]

Benchmark runs are scheduled for baseline = f749e1d and contender = cea5146. cea5146 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java