Add support for off-heap Map

[adam-wyluda]

It should support following basic operations:

• update - put element at given key
• get / apply - return underlying value in an Option
• contains
• remove
• foreach
• map
• mapValues
• transform
• transformValues
• toMap

Implementation could use ideas from non/debox Map: https://github.com/non/debox/blob/master/src/main/scala/debox/Map.scala