fix access to fields of vector members

fix access to fields of vector members

When ast_member encounters the result of an ast_entfield it
has to replace the ast_entfield's codegen as we cannot
evaluate the field access first.

We then perform the same action as ast_entfield but call
vectorMember on the field before issuing the load/address
instruction.

This effectively turns the codegen of the following ast
structure:
    member_of {
        field_of {
            entity,
            a_vector
        }
        memberid
    }
into the one of this structure:
    field_of {
        entity,
        member_of {
            a_vector
            memberid
        }
    }