Make Cache.Get allocation-free

[yiftachkarkason]

Changes

• Changed applyOptions and the Option[K,V] to be non-allocating by avoiding pointers
• Added the Test_Get_DoesNotAllocate to make sure that Get does not allocate (it fails before this commit)

Why
We encountered a performance issue when we heavily used the Cache, and narrowed it down to the allocation made by Cache.Get as part of the options processing.

Thank you for the great package!

[davseby]

Thank you for your contribution.

[swithek]

I think we can merge this? @davseby

Well done @yiftachkarkason

[davseby]

Following the package's naming, I think the allocsPerSingleRun would be a more appropriate function name. What is more, I don't see a point in using a named return here, the function scope is quite small, so something like this should be sufficient. Thanks, after this I will merge the PR. 🚀

Suggested change

	func allocsForSingleRun(f func()) (numAllocs int) {
	// `testing.AllocsPerRun` "warms up" the function for a single run before
	// measuring allocations, so we need do nothing on the first run.
	firstRun := false
	numAllocs = int(testing.AllocsPerRun(1, func() {
	if !firstRun {
	firstRun = true
	return
	}
	f()
	}))
	return
	}
	func allocsPerSingleRun(f func()) int {
	// `testing.AllocsPerRun` "warms up" the function for a single run before
	// measuring allocations, so we need to do nothing on the first run.
	var firstRun bool
	return int(testing.AllocsPerRun(1, func() {
	if !firstRun {
	firstRun = true
	return
	}
	f()
	}))
	}