Serialize and deserialize support for `Map` type.

[PopcornPaws]

It would be nice if Borsh-js would support Map types out of the box.

For example, I would like to use a rust BTreeMap type on the contract side that can be serialized/deserialized on the client side in a Map type. As far as I know, BTreeMaps are serialized similarly to a Vec, namely there is 4 bytes reserved at the front to encode the length (size) in LE representation, then the keys and values are serialized in a sorted sequence, so, contrary to a HashMap type, there's no danger of iterating over the key-value pairs randomly.

What do you guys think?

[zaxozhu]

@volovyk-s would you please take a look?

[volovyks]

@PopcornPaws awesome contribution! Have you tested it with borsh-rs? Can we serialize it here and deserialize it in Rust and vise-versa?

[PopcornPaws]

@PopcornPaws awesome contribution! Have you tested it with borsh-rs? Can we serialize it here and deserialize it in Rust and vise-versa?

@volovyk-s Yep, I tested the (de)serialization of a BTreeMap<[u8; 32], Pubkey>, a BTreeMap<String, Option<u32> and a BTreeMap<u16, String> type in Rust and it worked seamlessly in both ways.

[volovyks]

Thank you @PopcornPaws !

[PopcornPaws]

@volovyk-s thank you!