NormalizedString.clear() broken?

Hello. I think there are some problems with `NormalizedString` (tokenizers 0.15.2).

In the following example, `append()` works as expected.
```
from tokenizers import NormalizedString

s = NormalizedString("Hi.") # NormalizedString(original="Hi.", normalized="Hi.")
s.append("Hello.") # NormalizedString(original="Hi.", normalized="Hi. Hello.")
```

After using `clear()`, `append()` no longer modifies the `normalized` attribute.
```
from tokenizers import NormalizedString

s = NormalizedString("Hi.") # NormalizedString(original="Hi.", normalized="Hi.")
s.clear() # NormalizedString(original="Hi.", normalized="")
s.append("Hello.") # NormalizedString(original="Hi.", normalized="")
```

This is also a problem with `prepend`.