Map added to core libraries

[Turupawn]

This PR adds generic libraries centered around the Map structure and other types needed to implement it. Only additions are done, no code was removed.

Changes:

1. casting.fe: Added interfaces for casting, both upcast and downcast between two types.
2. codec.fe: Defines traits and implementations for encoding and decoding data to and from a buffer, using a cursor to read and write.
3. default.fe: Included the u256 default value of 0
4. map.fe: Added a generic MapStorage centered on the set and entry entry interface. Uses a customazible Hasher under the hood. This is wrapped by the Map type. Also supports generic arithmetic operations.
5. operators.fe: Basic arithmetic traits that can be used for method overloading in the future.

Still TODO/not covered by this PR:

• Casting imlementation is still TODO
• Through this PR I use a lot of u256 which is not ideal. Waiting for the implementation of higher kind types in the future to improve it
• The operators added on operators.fe should implement method overloading when the feature is added on fe.

This is currently being tested by the following two repos:

• fe-evm a fe library for the ethereum virtual machine
• fe-cookbook, fe examples. It currently has an ERC20 and ERC721 contracts