Ability to import and export maps

[kevcenteno]

I have a long running process that uses swissmaps to store values before they're committed to a datastore. In the event that the process is stopped, I would like to write a gob file with the swissmaps, and upon restart decode the gob file in to the swissmaps.

Currently, this is not possible because swiss.Map doesn't have any exported fields. To remedy this, I have to convert the swissmap to a regular map and then save it to the gob file, and on restart read the gob file maps and put the values into a swissmap.