Refactor tracking of open objects

[jhendersonHDF]

Rather than inserting an object's pointer value into a hash table for its particular type ID, we should instead hash the object's URI value and insert that value into the type ID's hash table instead. That way, when we need to check if an object is already opened, we can perform a single hash table lookup rather than needing to iterate through the hash table.

[mattjala]

Resolved in #102