* eword-decode.el (eword-encoded-word-prefix-regexp): New constant.

* eword-decode.el (eword-encoded-word-prefix-regexp): New constant.
(eword-encoded-word-suffix-regexp): New constant.
(eword-encoded-text-in-unstructured-regexp): New constant.
(eword-encoded-word-in-unstructured-regexp): New constant.
(eword-after-encoded-word-in-unstructured-regexp): New constant.
(eword-decode-entire-string): New function.
(eword-decode-unstructured): Use `eword-decode-entire-string'.
(eword-decode-comment): Use `eword-decode-entire-string'.
(eword-decode-quoted-string): Use `eword-decode-entire-string'.
(eword-decode-string): Add optional argument `default-mime-charset'.
(eword-decode-region): Add optional argument `default-mime-charset'.
(eword-decode-header): Refine `code-conversion' treatment.