Generic acceptance tests for the fungible::* API, so any implementation can check it does the right thing.

Fungible API

• Regular: Inspect and Mutate traits fungible conformance tests: Inspect and Mutate substrate#13852
• Regular: Unbalanced and Balanced traits Unbalanced and Balanced fungible conformance tests, and fungible fixes #1296
• Hold traits
• Freeze traits

Fungibles API

• Write a new task list here describing TODOs to test fungibles. Should reuse as much code from the fungible api tests as makes sense (see fungible/item_of.rs)