Removng alternate proposal from layout of types and offsets for unions in layout.md

[Micah Kornfield]

The current layout.md lists an alternate proposal for the layout of of
these values:

"Alternate proposal (TBD): the types and offset values may be packed into an
int48 with 2 bytes for the type and 4 bytes for the offset."

Any objections to removing this proposal and moving forward with
keeping them as two separate arrays?

Thanks,
Micah