[Gas Metering] Add storage_rebate to Object and introduce gas price

[lxfind]

This PR does two things together, since they are tightly coupled.
What we are trying to do here is to properly implement Alonso's storage rebate design:
Whenever an object is being mutated, what we do is to first think that this original object is being deleted, and hence we are giving back a storage rebate, and then the new version of the object is being inserted, which will charge a new storage cost.
Because we want to make sure that the storage fund never gets depleted, the storage rebate on the same object can never be bigger (in SUI) than the storage cost when the object was added. To achieve this, we need to track the amount of SUI the previous mutation paid for the storage, and this amount will be used as the rebate next time this same object is being mutated.
This PR achieves this by adding these two things:

1. We are initializing the gas status with a computation gas unit price and a storage gas unit price. This will eventually come from epoch metadata but I haven't added that part.
2. We are adding a storage_rebate field to Object. This value represents the amount of rebate (in SUI) someone will get if they ever delete this object from storage. It's set by the previous transaction that mutated this object. In this implementation, what we do is that every time when an object is being mutated (not just deleted), we first "delete" the old object and provide the storage rebate, and then charge the storage cost based on the current storage gas unit price.

[sblackshear]

Could take Object to avoid the clone at 133 i the caller happens to have ownership?

[lxfind]

We cannot unfortunately. We still need to actually deduct gas from the gas_object after this function returns.

[sblackshear]

Is it safe to instead do

if object.is_read_only() { continue }

at the beginning of the loop, or even at the beginning of the function? At least based on the name, it sounds like charge_gas_for_storage_changes should never be called for immutable objects.

[lxfind]

It's not. Newly published package will show up in self.written as well, and we need to charge for storage too.

[codecov]

Codecov Report

Merging #1254 (8bdc983) into main (640dcbe) will increase coverage by 0.11%.
The diff coverage is 98.97%.

❗ Current head 8bdc983 differs from pull request most recent head 2f6d9eb. Consider uploading reports for the commit 2f6d9eb to get more accurate results

@@            Coverage Diff             @@
##             main    #1254      +/-   ##
==========================================
+ Coverage   81.72%   81.84%   +0.11%     
==========================================
  Files          99       99              
  Lines       20613    20683      +70     
==========================================
+ Hits        16847    16929      +82     
+ Misses       3766     3754      -12     

Impacted Files	Coverage Δ
sui_core/src/execution_engine.rs	100.00% <ø> (ø)
sui_types/src/gas.rs	95.91% <97.50%> (-4.09%)	⬇️
sui_core/src/authority.rs	94.95% <100.00%> (ø)
sui_core/src/authority/temporary_store.rs	83.52% <100.00%> (+2.03%)	⬆️
sui_core/src/gateway_state.rs	91.33% <100.00%> (ø)
sui_core/src/unit_tests/authority_tests.rs	87.38% <100.00%> (-0.07%)	⬇️
sui_core/src/unit_tests/consensus_tests.rs	96.90% <100.00%> (-0.11%)	⬇️
sui_core/src/unit_tests/gas_tests.rs	97.60% <100.00%> (+0.11%)	⬆️
...ammability/adapter/src/unit_tests/adapter_tests.rs	98.55% <100.00%> (ø)
sui_types/src/object.rs	81.49% <100.00%> (-1.07%)	⬇️
... and 8 more

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