Rust

Introduce the `Env::nested_read_txn` from an `RwTxn` #644

Job	Run time
Test the heed project (macos-latest)	1m 1s
Test the heed3 project (ubuntu-latest)	54s
Test the heed3 project (macos-latest)	1m 14s
Test the heed3 project (windows-latest)	2m 37s
Test the heed project (windows-latest)	2m 28s
Test the heed project (ubuntu-latest)	49s
Check the heed3 project (ubuntu-latest)	36s
Ensure heed3 is not erasing heed	4s
Check the heed3 project (macos-latest)	53s
Check all the features of the heed3 project	57s
Check all the features of the heed project	1m 58s
Run the heed examples (ubuntu-latest)	53s
Run the heed examples (macos-latest)	1m 2s
Run the heed3 examples (ubuntu-latest)	33s
Ensure the heed and heed3 project are formatted	15s
Ensure clippy is happy on heed and heed3	39s
Check the heed3 project (windows-latest)	1m 36s
Run the heed3 examples (macos-latest)	44s
	19m 13s

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