Multiple changes.

- 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.

FossilOrigin-Name: e1ed7bf613981607bb3b57ce7dd3e58b94ea3046e140b6dc37440da8d2909f94

.pryrc

@@ -13,4 +13,3 @@ end
 
 db = MDBX::Database.open( 'tmp/testdb', max_collections: 100 )
 
-