IArray: from_iter optimization for empty arrays

[cecton]

This will optimize creation of IArray from an empty iterator.

Also added a constant IArray<T>::EMPTY.

[kirillsemyonkin]

Does Rust not optimize &[] to 0? Wonder if profiling before doing this is needed, if its only for optimization and not for making API nicer.

[cecton]

hmm it's true that a collection of 0 elements collected to a vec will not allocate. But it does make an Rc for nothing I guess

[kirillsemyonkin]

If we have no quick way to benchmark these changes, it is fine then lol

[cecton]

I copy-pasted the divan thingy from yew to see and oh wow it's terribly faster:

After:

vnode                           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ from_iter_empty_collections  2.919 ns      │ 4.163 ns      │ 2.933 ns      │ 2.944 ns      │ 100     │ 1000000000

Before:

vnode                           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ from_iter_empty_collections  16.53 ns      │ 18.01 ns      │ 16.76 ns      │ 16.77 ns      │ 100     │ 1000000000

(I need to rename the test file I know :D)

[cecton]

The one for single element is also impressive:

Before:

Timer precision: 30 ns
vnode                                        fastest       │ slowest       │ median        │ mean          │ samples │ iters
├─ from_iter_collection_with_single_element  24.04 ns      │ 25.43 ns      │ 24.2 ns       │ 24.21 ns      │ 100     │ 1000000000
╰─ from_iter_empty_collections               15.4 ns       │ 16.27 ns      │ 15.71 ns      │ 15.81 ns      │ 100     │ 1000000000

After:

Timer precision: 30 ns
array                                        fastest       │ slowest       │ median        │ mean          │ samples │ iters
├─ from_iter_collection_with_single_element  9.718 ns      │ 12.57 ns      │ 9.756 ns      │ 9.787 ns      │ 100     │ 1000000000
╰─ from_iter_empty_collections               3.142 ns      │ 3.153 ns      │ 3.15 ns       │ 3.15 ns       │ 100     │ 1000000000

[cecton]

I just noticed that Rc has a from_iter to Rc<[T]>. This should also avoid one allocation.

array                                        fastest       │ slowest       │ median        │ mean          │ samples │ iters
├─ from_iter_collection_with_single_element  19.36 ns      │ 21.4 ns       │ 19.49 ns      │ 19.51 ns      │ 100     │ 1000000000
╰─ from_iter_empty_collections               12.11 ns      │ 12.2 ns       │ 12.16 ns      │ 12.16 ns      │ 100     │ 1000000000

(The others are better but it's good to have it for collections that have more than 1 items!)

[cecton]

@kirillsemyonkin I don't really want to do all the CI stuff for now to display a comment with the benchmark -_- but I'm already happy with the results of the benchmarks. If it's good for you I will merge.