chore: apply modular approach to fungible example contract

[ndkazu]

Description

Issue #303
Simplify the contract code by using a modular design, similar to what is shown here: https://github.com/Cardinal-Cryptography/PSP22/blob/38e0b72ab165dc13cd420d9918dff7ae8c931021/lib.rs#L3-L7

Method

Create a trait Api in a different file traits.rs with all the function calls using api:: in lib.rs

[ndkazu]

Work in progress. First commit is already implementing the method described in some parts.

[chungquantin]

Hi @ndkazu, thanks for making progress on this. I would say creating another example fungile-modular in the same folder is better. But let's keep the changes made to the fungible example for the code review purpose and refactor after it is approved.

[chungquantin]

Discussed with the team and we decided that the issue for modular approach is no longer needed. My suggestion if you are still interested to contribute is to craft a few examples for the PSP22 contract: #337 . Let me know if you need any help!