Clean up the ERC20 contract

[RitaMAllenCA]

Currently, the ERC20 test contract in the ERC20 Repo contains unsafe code. The below comments from the contract contains the details.

The aim of this ticket is to analyse the contract, remove unsafe code and update the example contract to align to Casper's Token Standards.

Comments in the example contract;

• For security reasons never use this entrypoint definition in a production contract. This is marks the entry point as public.
• This is unsafe and this test contract is not a ERC20 token standard-compliant token.

This can all be addressed if we flatten ERC20 implementation in the style of the cep-78 implementation i.e. a simple contract and no library.

the deliverable should be a contract repo with one straightforward contract implementation and integration tests