||Make Unit Headers use less space
Section 7.5.1, pg 199
This is an alternative for issue 161031.2. Instead of making the
header size/fields completely depend on the unit type just use some
bits to describe whether or not a unit header has any of the option
fields. This could be as simple as dedicating just the low 6 bits to
the actually unit type and use the upper two bits to indicate whether
the header has an (8 byte) ID field and/or an (4 or 8 byte) DIE offset field.
This allows 64 unit types and makes it easy to describe which optional
fields are in the header for currently unknown new types without wasting
any padding space.