(eword-decode-structured-field-body): New implementation; abolish

(eword-decode-structured-field-body): New implementation; abolish
optional argument `must-unfold'; delete DOC-string.
(eword-decode-and-unfold-structured-field-body): Renamed from
`eword-decode-and-unfold-structured-field'; delete DOC-string.
(eword-decode-and-fold-structured-field-body): Renamed from
`eword-decode-and-fold-structured-field'; abolish optional argument
`must-unfold'; delete DOC-string.
(eword-decode-unstructured-field-body): Abolish optional argument
`must-unfold'; delete DOC-string.
(eword-decode-and-unfold-unstructured-field-body): Renamed from
`eword-decode-and-unfold-unstructured-field'; delete DOC-string.
(eword-decode-unfolded-unstructured-field-body): New function.