Don't memoize strings when hashing since two identical strings may have different python ids
When hashing an object that has several times the same string, the hashing could return a different hash if the identical strings share the same python `id()` or not.

Here is an example code that shows how the issue can affect the caching:
```python
import json
import pyarrow as pa
from datasets.features import Features
from datasets.fingerprint import Hasher

schema = pa.schema([pa.field("some_string", pa.string()), pa.field("another_string", pa.string())])
features_from_schema = Features.from_arrow_schema(schema)
Hasher.hash(features_from_schema) # dffa9dca9a73fd8c

features_dict = json.loads('{"some_string": {"dtype": "string", "id": null, "_type": "Value"}, "another_string": {"dtype": "string", "id": null, "_type": "Value"}}')
features_from_json = Features.from_dict(features_dict)
Hasher.hash(features_from_json) # 3812e76b15e6420e

features_from_schema == features_from_json # True
```

This is because in `features_dict`, some strings like "dtype" are repeated but don't share the same id, contrary to the ones in `features_from_schema`.

I fixed that by disabling memoization for strings.

This could be optimized in the future by implementing a smarter memoization with a special handling for strings.
This change slows down the hash computation a little bit but from my tests it doesn't look too impactful. So I think it's fine to merge this.