Changes

Update camlinternalMod and Obj.new_block for new closure representation (#9691)

* camlinternalMod: use closure metadata for copying a closure over another

This change is careful to avoid writing a value into what was
previously a raw field or conversely, clearing fields that change
category first.

We also clear the end of the block, to make it easier to reason about
lifetime of values that could have been referenced there. (We don't
expect this to make a different in practice.)

* Obj: new submodule Closure giving basic access to closure metadata

* Ensure that Obj.new_block returns a sensible uninitialized closure

* Changes
 (commit: bdbf5c3)