- Alter the behavior of #clear, so it doesn't destroy collection
environments, but just empties them.
- Add #drop, which explictly -does- destroy a collection environment.
- Run all cursor methods through rb_protect, to ensure proper
cursor cleanup in the event of an exception mid iteration.
- Fix the block form of collections to support multiple scopes.
- Switching to a collection now automatically creates its environment.
- Add include? and has_key?, for presence checks without allocating
value memory or requiring deserialization.
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