static deltas: Use OtVariantBuilder to create deltas

static deltas: Use OtVariantBuilder to create deltas

This allows us to create the final delta desciptor directly on disk
rather than having it all in memory. This is nice because it can
become quite large if inlined parts are used.

Note however, that we currently generate all the delta parts in
memory before adding them to the delta, so we still keep all individual
parts in memory. Fixing that is the next step.

Closes: #1309
Approved by: cgwalters