add size() function to all Mask classes

this is for STL container compat

Signed-off-by: Matthias Kretz <kretz@kde.org>
Mask: complete member types

- new Vector member type that refs back to Vector<T>
- fix VectorEntryType for AVX::Mask
- add VectorType for Scalar::Mask

Signed-off-by: Matthias Kretz <kretz@kde.org>