feat (pop api): implement asset management interfaces local fungibles

[Daanvdplas]

Implement the functionality regarding Asset Management:

• create
• start_destroy
• destroy_accounts
• destroy_approvals
• finish_destroy
• set_metadata
• clear_metadata

An open question here is whether creating assets should be put in the constructor of a contract using the local fungibles api or not. In other words, no contract will create an asset other than in the contract. I'm not sure about this at the moment and needs to be looked at. As a result of this, example contracts should be made that help developers understand how to manage assets in their contracts using the api (#108).

Include integration-tests.

[Daanvdplas]

Closed by #151