NFT cannot be transferred back to the original chain if ClassId contains the IBC separator

[taramakage]

As pointed out by @giansalex in gon-evidence#245, an NFT under a Class with a ClassId that contains the IBC separator / cannot be transferred back to its original chain.

This is because types.ParseClassTrace() assumes that there is no slash in the base ClassId. As a result, it processes p/0/1 as the path p/0 and the base ClassId 1.

To address this issue, we could:

1. either modify the spec and separate the ClassId field into ClassTrace and BaseClassId fields, and distinguish them without relying on a single slash.
2. or restrict the use of slashes in ClassId.