Add support of `len` to `collections.deque`

Currently `collections.deque` in micropython doesn't support `len`:
```python
>>> from collections.deque import deque
>>> q = deque()
>>> len(q)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'deque' has no len()
```
But cpython supports:
```python
In [1]: from collections import deque
In [2]: q = deque()
In [3]: len(q)
Out[3]: 0
```
So I've added it to micropython `collections.deque`.