RFC: make all inventories immutable except via deltas

[Robert Collins]

I wonder if we have enough support in our inventory delta code to:
 - make all InventoryEntries immutable
 - remove mutators on Inventory other than apply_delta

This might make it easier to avoid aliasing and similar bugs with
Inventory, and would make it possible for dirstate to keep its created
inventory up to date too. (By having that inventory not have an
apply_delta method - or having the apply_delta apply to the dirstate
too).

-Rob
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