Add QByteArrayView::toHex() & toBase64()

QByteArrayView has convenient toInt(), toFloat(), etc, that can be used on sliced() "non-copy" of byte array.

But toHex() is missing though, and it might help to save even bigger copies.

For example, some real code (Qt5):

imei = m_data.mid(4, 8).toHex();

could be converted to:

imei = QByteArrayView{m_data}.sliced(4, 8).toHex();

to save redundant copy.