Vector generator

Vector generators represents the domain of vectors of a given size with all the elements of the same type.

Examples:
```php
Array
(
    [0] => 2
    [1] => 5
    [2] => 10
    [3] => 3
    [4] => 8
    [5] => 6
    [6] => 7
    [7] => 9
    [8] => 1
    [9] => 7
)
Array
(
    [0] => 3
    [1] => 9
    [2] => 3
    [3] => 10
    [4] => 8
    [5] => 8
)
Array
(
    [0] => 3
    [1] => 6
    [2] => 3
    [3] => 5
    [4] => 4
    [5] => 8
    [6] => 9
    [7] => 10
    [8] => 4
    [9] => 6
)
```

Shrinking is done on a random number of elements, here an example of shrinking on a Vector of Natural numbers:
```php
Array
(
    [0] => 4
    [1] => 10
    [2] => 2
    [3] => 3
    [4] => 8
)
Array
(
    [0] => 3
    [1] => 10
    [2] => 2
    [3] => 3
    [4] => 8
)
Array
(
    [0] => 3
    [1] => 10
    [2] => 1
    [3] => 3
    [4] => 8
)
Array
(
    [0] => 2
    [1] => 10
    [2] => 1
    [3] => 2
    [4] => 8
)
Array
(
    [0] => 1
    [1] => 10
    [2] => 0
    [3] => 2
    [4] => 8
)
Array
(
    [0] => 1
    [1] => 10
    [2] => 0
    [3] => 1
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 9
    [2] => 0
    [3] => 1
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 9
    [2] => 0
    [3] => 1
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 9
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 8
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 7
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 7
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 7
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 7
    [2] => 0
    [3] => 0
    [4] => 8
)
Array
(
    [0] => 0
    [1] => 7
    [2] => 0
    [3] => 0
    [4] => 8
)
```
Yes, I took the inspiration from Triq. We can be more strict about this but at the end of the day it shrinks quite always to a lower element of the domain.
I think that only by using it for real we can tune the shrinking strategies.