Add simplest possible left and right "stairs" converters for sequences.

Nobody has claimed this yet.



Do not use ∅, pictures are wrong.

## The correct source example is:

```
[1, 2, 3, 4]
```

## Two ways to transform this into sequences

## DescendingVariantConverter

This converter starts with a pair of two first elements and the goes "down the hill".
```
(((1 2) 3) 4)
```

Where `(1 2)` means, that it's ID used to make next pair and so on.

```
var doubletId = _links.GetOrCreate(sequence[0], sequence[1])
```

## AscendingVariantConverter

In that case, converter starts from the end of list/sequence/array.

The complete result will be:

```
(1 (2 (3 4)))
```

Where `(3 4)` means, that it's ID used to make next pair and so on.

```
var doubletId = _links.GetOrCreate(sequence[i-1], sequence[i])
```

## P.S.

Both sequences should be created at `./csharp/Platform.Data.Doublets.Sequences/Converters` folder.

Start by inspecting the ./csharp/Platform.Data.Doublets.Sequences/Converters folder and the sequence examples in this issue. Implement both converters so [1, 2, 3, 4] produces the specified descending and ascending nested pair forms, without using ∅ or the incorrect pictures.

Written by the indexing model from the issue text.