Try to get rid of APIs that use raw 'const {char, QChar} *, length'
pairs. Instead, use QByteArrayView or QStringView.
As QStringConverter is a new class, simply change the API to what we'd like
to have. Also adjust hidden API in QStringBuilder and friends.
Change-Id: I897d47f63a7b965f5574a1e51da64147f9e981f6
Reviewed-by: Lars Knoll <lars.knoll@qt.io>