`static` method support in builtin functions and classes.

**Describe the solution you'd like**
What we have now.
```radon
json = Json()
json.loads(...)
json.dumps(...)
```
What it should be.
```radon
Json.loads(...)
Json.dumps(...)
```
For this we can create a new decorator `@static` and this will convert that method as static so it can be accessible by Radon as static method.