CSV and other text formats may come with line sizes >64k that so far were
truncated due to limitations in ByteString/UniString/String, even if one line
consists of several fields that each are <64k.

Introduced additional SvStream methods that read into rtl::OString and
rtl::OUString and let SvStream::ReadUniOrByteStringLine() fill solely an
rtl::OUString.

Made Calc CSV import use those.