Answer a new unicode string instance from the receiver.

     Examples:
        'Grinning Face (U+1F600)'.
        self assert: [(Utf32LE with: 16r1F600) asUnicodeString first name = 'GRINNING FACE'].

        'Invalid UTF32-encoding'.
        self assert: [[(Utf32LE with: 16rD800) asUnicodeString. false] on: Exception do: [:ex | ex exitWith: true]].

        'Repaired invalid UTF8-encoding'.
        self assert: [((Utf32LE with: 16rD800) asUnicodeString: true) = UnicodeScalar replacementCharacter asUnicodeString].

     Answers:
        <UnicodeString>

  Answer true if the content of the container represents valid UTF-32 data
     encoded in the endian type defined by the container.

     Examples:
        'Grinning Face (U+1F600) in platform endian'.
        self assert: [
            System bigEndian
                ifTrue: [(Utf32 with: 16r1F600) isValid]
                ifFalse: [(Utf32 with: 16r1F600) isValid]].

        'Invalid because of isolated surrogate - 16rD800'.
        self assert: [(Utf32LE with: 16r1F600 with: 16rD800) isValid not]

     Answers:
        <Boolean> true if valid, false if invalid