Rust

Introduce the `Env::nested_read_txn` from an `RwTxn` #457

Job	Run time
Ensure clippy is happy on heed and heed3	24s
Check the heed3 project (ubuntu-latest)	14s
Check all the features of the heed project (ubuntu-latest)	16s
Check the heed3 project (windows-latest)	1m 46s
Check all the features of the heed3 project (ubuntu-latest)	19s
Check all the features of the heed project (macos-latest)	21s
Check the heed3 project (macos-latest)	27s
Run the heed examples (ubuntu-latest)	26s
Check all the features of the heed3 project (macos-latest)	24s
Run the heed examples (macos-latest)	30s
Ensure the heed and heed3 project are formatted	12s
Run the heed3 examples (ubuntu-latest)	28s
Test the heed3 project (ubuntu-latest)	41s
Test the heed project (ubuntu-latest)	40s
Run the heed3 examples (macos-latest)	26s
Test the heed3 project (windows-latest)	45s
Ensure heed3 is not erasing heed	4s
Test the heed project (windows-latest)	1m 45s
Test the heed3 project (macos-latest)	32s
Test the heed project (macos-latest)	39s
	11m 19s

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