Add more failing tests
Share the memory store among all store instances.

This fixes the S.create function which had too many non-shared value store.
Add a 'contents' function to expose all the store contents.

We will want to limit who can call this function in the future (but for now on it's pretty useful to debug)
Add a comment about the difference of semantics between A.list and M.list

The former only return the given key, the latter returns all the available keys. This is far from ideal, this should be fixed at one point (See #5).
Add some debug statements
Simplify the Graph API, use it to dump the contents of the database as a graphviz graph.
Fix IrminMisc.pretty_list
Type the marshaling elements and do not crash if reading something whose type is not the one we want.

That's pretty useful when you are scanning a store to dump its elements.
Factor out some code
Start implementing import/export (this is still broken)
Update tests