Refactor helpers for PyUnicodeErrorObject.

- Unify `get_unicode` and `get_string` in a single function.
- Allow to retrieve the underlying `object` attribute and its
  size in one round.
- Use a common implementation for the following functions:

  - `PyUnicode{Decode,Encode}Error_GetEncoding`
  - `PyUnicode{Decode,Encode,Translate}Error_GetObject`
  - `PyUnicode{Decode,Encode,Translate}Error_{Get,Set}Reason`
  - `PyUnicode{Decode,Encode,Translate}Error_{Get,Set}{Start,End}`

Author: picnixz