Add ofEntries static method to Map interfaces

[viciscat]

Pretty simple PR, I think Things to note:

• Java doc is a bit quaint because I thought it was useless to copy paste the original methods
• an entry(key, value) static method was also added to make it easier to use, just like java.util.Map
• I did not include Int2IntMapTest because I would have put the same things I did in Object2ObjectMapTest
• More tests could probably be added but I have no idea what they could be

Close #329