feat: improve currency typing

[benjlevesque]

This change allows 2 things:

• using more specific types, like ERC20Currency:

const method = (c: ERC20Currency) => c;
if (curr.type === 'ERC20') {
  method(curr);
}

• simplify the use of network:

if(curr.type === 'ISO4217'){
  // type: undefined
  curr.network
} else{
  // type: string
  curr.network
}