Upstream changes to Aave v3 to make it more extensible

[davidlaprade]

We already did this here: https://github.com/aave/aave-v3-core/pull/726

Other functions we should update:

• aToken.burn to make it public (rather than external)
• aToken.intialize to make it virtual (so that we can override flex voting aTokens to self-delegate during initialization)
• aToken._transfer to make it virtual (so that we can checkpoint during both transfer and transferFrom with the same code)

[davidlaprade]

I realized this morning that we don't need to make aToken.burn() public to do what we want. See https://github.com/ScopeLift/flexible-voting/pull/21/commits/ffc286730a460d64d763e9075adb853a4faf1046 for a better solution that didn't require any Aave changes.

I pushed a PR to make the other two changes: https://github.com/aave/aave-v3-core/pull/774

[apbendi]

@davidlaprade should we close this now that our PR has been merged by Aave?