Implement Map type

[jwhb]

The schema::Map type is not implemented yet. It's structure and functionality will mostly follow schema::Set.

nftables-rs/src/schema.rs

Lines 198 to 201 in 720d2c7

	#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
	pub struct Map {
	// TODO
	}

Documentation for Set/Map: https://www.mankier.com/5/libnftables-json#Ruleset_Elements-Set_/_Map

A test should be added, like here.

[jwhb]

Implemented in #11. Thanks @marcinosypka!