General repr format for our internal ExtensionArrays
Triggered by the discussion I am having in https://github.com/pandas-dev/pandas/pull/22511/, I was also thinking we should look at our EAs reprs.

Currently we have for IntegerArray:

```
In [14]: pd.core.arrays.integer_array([1, 2, 3, None])
Out[14]: IntegerArray([1, 2, 3, nan], dtype='Int64')
```

which *looks* like code, but actually is not valid code (because the constructor needs an array, has no dtype argument, and nan is actually no defined).

So also here (since here we still have the freedom to choose something without the concern of changing something), I think we should have some discussion about what we ideally want.