Add support for 128 bit HashMap key serialization

[Mrreadiness]

Now HashMap doesn't support i128/u128 key serialization (deserialization works).
Example of code:

let mut b = HashMap::new();
b.insert(0_i128, String::new());
serde_json::to_value(b).unwrap();

Error:

Error("i128 is not supported", line: 0, column: 0)

I don't find any reason why. So I added the missing methods.

[dtolnay]

Thank you!