Multiple changes.

- Complete first round of documentation.
  - Complete first round of tests and coverage.
  - Expand the thread benchmarker for testing metasync.
  - Add enumerators (each_key/each_value/each_pair) using cursors.
  - Remove keys() implementation in favor of using the emumerable.
  - Make deserialization more DRY.
  - Add an efficient length() method.
  - Add various Hash-alike methods.
  - General code cleanup for release.

FossilOrigin-Name: 0d2bd3995f203c9ac1734ac3da32dd2f09efda9a394e554e6006e44dd07a33b0

.pryrc

@@ -11,6 +11,6 @@ rescue Exception => e
 		e.backtrace.join( "\n\t" )
 end
 
-# db = MDBX::Database.open( 'tmp/testdb' )
+db = MDBX::Database.open( 'tmp/testdb', max_collections: 100 )