Rust

Introduce the `Env::nested_read_txn` from an `RwTxn` #646

Job	Run time
Test the heed3 project (windows-latest)	2m 23s
Test the heed3 project (ubuntu-latest)	53s
Test the heed3 project (macos-latest)	52s
Test the heed project (ubuntu-latest)	50s
Test the heed project (windows-latest)	2m 9s
Test the heed project (macos-latest)	1m 1s
Check all the features of the heed3 project	2m 6s
Run the heed examples (ubuntu-latest)	54s
Run the heed3 examples (macos-latest)	37s
Run the heed3 examples (ubuntu-latest)	30s
Check the heed3 project (windows-latest)	1m 22s
Run the heed examples (macos-latest)	1m 26s
Check the heed3 project (ubuntu-latest)	37s
Check the heed3 project (macos-latest)	37s
Ensure the heed and heed3 project are formatted	16s
Ensure heed3 is not erasing heed	3s
Check all the features of the heed project	2m 18s
Ensure clippy is happy on heed and heed3	40s
	19m 34s

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