Rust

Introduce the `Env::nested_read_txn` from an `RwTxn` #458

Job	Run time
Ensure clippy is happy on heed and heed3	25s
Check all the features of the heed project (ubuntu-latest)	14s
Check the heed3 project (ubuntu-latest)	13s
Check all the features of the heed3 project (ubuntu-latest)	26s
Check all the features of the heed project (macos-latest)	21s
Check all the features of the heed3 project (macos-latest)	25s
Check the heed3 project (windows-latest)	2m 26s
Run the heed examples (ubuntu-latest)	34s
Check the heed3 project (macos-latest)	22s
Run the heed examples (macos-latest)	40s
Ensure the heed and heed3 project are formatted	15s
Run the heed3 examples (ubuntu-latest)	23s
Run the heed3 examples (macos-latest)	25s
Test the heed3 project (ubuntu-latest)	38s
Ensure heed3 is not erasing heed	2s
Test the heed3 project (windows-latest)	3m 8s
Test the heed project (ubuntu-latest)	31s
Test the heed3 project (macos-latest)	37s
Test the heed project (windows-latest)	2m 55s
Test the heed project (macos-latest)	36s
	15m 36s

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