valuetypes constrains

[luke-titley]

This is a proposal for a constraint I think we will need to have on value types.

'A valuetype record can only have fields that are also valuetype or opaquebytes'. It should be an error either in astgen or in asttoc if a user marks a record as valuettype but one of it's fields is opaqueptr.

The reason for this constrain is that it's the only way we can guarantee that the c struct has a one for one memory layout of it's c++ equivalent.

The effect of this constraint is that a record containing stl types cannot be exposed as valuetype.