[Execution] Forest.Read() can use less memory

[fxamacker]

Problem

Forest.Read() reads payloads from MTrie and makes a deep copy of both payload key and value. Forest.Read is only called by Ledger.Get . Ledger.Get creates a new slice of payload values and discards payload keys.

Deep copying payload key is unnecessary and it creates 4 heap allocations for each key.

Updates epic #1744

The Proposed Solution

Change Forest.Read to only deep copy and return payload values. This change is limited to Forest in ledger package.

[fxamacker]

Closed by PR #2477. Not sure why it wasn't closed automatically.