API refactor: Describe a Forwarding Path

[0xGabi]

This issue goes further into the details of what it takes to create a Forwarding Path description. The logic that we currently support was migrated from aragon.js and it's not flexible enough to provide lot of value for custom UI experiences.

To transition into a more powerful and descriptive data type as the one described in #132 we need to address a few tasks:

• Refactor the core logic that handles how the ForwardingPathDescription will be constructed: https://github.com/aragon/connect/blob/master/packages/connect-core/src/utils/descriptions.ts#L14
• Refactor the logic that given an evm script construct a ForwardingPathDescription: https://github.com/aragon/connect/blob/master/packages/connect-core/src/utils/path/decodePath.ts#L14
• Refactor how we use Radspec to provide more flexibility and control to Connect users
• Consider moving the helpers and utilities to encode and decode descriptions to their own Connect package