Change DiploidMetadata to incorporate a unique individual ID

[molpopgen]

This is the first step in improving how we do pedigree recording.

[molpopgen]

I now believe this to be unnecessary:

• It is a big change to the C++ back end.
• The changes break the tskit metadata format
• The changes need O(N) memory to track info that we lose,
  eliminating the potential gains.

A better path forward is to build nice pedigree data
from our current individual metadata.
Doing so is not hard, but what is missing is good API support.
We should just provide that support.