Add generic key-value support for Map

[HoKim98]

Hello, this PR supports Map<K, V>.

Generic maps are useful for deserializing maps (i.e. impl<'de> Deserializer<'de, K, V> for Map<K, V>) into other types.

Side-effects

The map Map<K, V> implements other map-like operations and traits.
It may ask for users to specify the types K, V.

let mut map = serde_json::Map::new();  // error[E0282]: type annotations needed for `serde_json::Map<_, _>`
assert_eq!(map.entry("serde").key(), &"serde");

Instead, I set the default type of Map to Map<String, Value>.

let mut map: serde_json::Map = serde_json::Map::new();
assert_eq!(map.entry("serde").key(), &"serde");

[dtolnay]

Thanks for the PR. I would prefer to keep the less generic implementation so that users are not forced to write type annotations so often when using serde_json::Map. This type is only intended for representing a map deserialized from JSON. For any other map, please use a different map type.

[HoKim98]

Then what about renaming Map to other one (e.g. GenericMap) and make an alias for Map (i.e. type Map = GenericMap<String, Value>;)? It minimizes the side effect because there is few usage of generic Map<K, V> for now (nothing implemented at all).

[dtolnay]

I prefer not to support that in this crate. There are enough other map types that are already usable for that purpose.