Decimals on ERC20

[martriay]

Considering that:

• In the solidity implementation of the contract the value is hardcoded at 18 and the way to change it is through overloading, which is not available in Cairo.
• According to EIP20 decimals' type should be uint8, which is way smaller than Cairo's felt. This could cause problems when bridging tokens to EVM compatible chains.

Should we then add a new parameter to the constructor and sanitize it's under 2^8?

[frangio]

Most tokens should use 18, so I think we should just set it to 18.

[martriay]

Yeah but some don't (e.g. USDC uses 6). We should aim for maximal compatibility.