Clarify properties of map format family #210

It is not clear from the specification whether map is supposed to be ordered sequence or unordered. From the same language being used for both array and map, I'd say it is ordered

Array format family stores a sequence of elements in 1, 3, or 5 bytes of extra bytes in addition to the elements.
Map format family stores a sequence of key-value pairs in 1, 3, or 5 bytes of extra bytes in addition to the key-value pairs.

I think it will be beneficial to explicitly define it as one or the other.

It would also be a good idea to have both types of maps in the specification, because many languages have both ordered and unordered collections and so they can deserialize into correct type.

If you prefer to keep only one map type, then ordered version will allow to store both types without loss of information (I know it is de-facto ordered, but I think it should be explicitly specified).