Consider to introduce new zserio type for offsets

[mikir]

It would be more convenient for users, if Zserio will support directly the offset type:

struct Schema
{
    offset32 myOffset;
    uint8 someValue;
    varsize someVariableSizedValue;
myOffset:
   string stringValue;
};

The following types should be supported:

• offset16 with the underlying type uint16
• offset32 with the underlying type uint32
• offset64 with the underlying type uint64

However, the old syntax should be still valid (uint16 myOffset) because of the backward compatibility.

For more info, please have a look to the issue #640.