Optimize some iterations in BodyExtractor and BodyInserter

[yuzawa-san]

Using iterators for trivial findFirst and toList cases uses less CPU and memory (in these cases, not while costing readability)
Here is an icicle graph example of the CPU usage before

and here it is after:

There was about a 10-15% overhead with the streams usage, which we can save on by doing this.

[simonbasle]

indeed these kind of stream usage can get an order of magnitude slower especially for small collections... this looks good to me overall, thanks for the PR @yuzawa-san 👍
one minor comment to address from a readability standpoint and I think we can merge this in time for 6.0.8

[simonbasle]

compared to the BodyExtractor case, I find that cast call is a little too buried among all these parameters, making it less visible. would you mind extracting it to an intermediate local variable? this should be efficiently optimized away by the JIT compiler, so that is purely from a readability perspective.

[yuzawa-san]

@simonbasle updated with intermediate variable.

#29972 also gets around another (I would say more expensive) streams usage in ReadOnlyHttpHeaders.

spring-framework/spring-web/src/main/java/org/springframework/http/ReadOnlyHttpHeaders.java

Lines 150 to 156 in 4e896c8

	@Override
	public Set<Entry<String, List<String>>> entrySet() {
	return this.headers.entrySet().stream().map(SimpleImmutableEntry::new)
	.collect(Collectors.collectingAndThen(
	Collectors.toCollection(LinkedHashSet::new), // Retain original ordering of entries
	Collections::unmodifiableSet));
	}

I left this one there because that PR avoids that entrySet and also I have been working on a branch where I got ReadOnlyHttpHeaders to use CollectionUtils.unmodifiableMultiValueMap internally.

[simonbasle]

thanks for the change @yuzawa-san 👍