rpcrt4: Factorise conformant array NDR functions and use them to fix the wire-representation of complex structures with conformant arrays.

Factorise each conformant array function into the part that deals with
reading, writing or sizing the conformance and another part that deals
with reading, writing or sizing the variance and the element data.

This allows complex structures to use the right wire format where the
conformance appears before the structure data starts.