tapgarden: key asset metas and proofs by locator instead of script key

[jharveyb]

Right now one of the invariants we maintain for a minting batch is that seedling names are unique. This means that multiple seedlings should be able to use the same script key.

However, we currently key some data structures inside the minter only on the asset script key, which means we could have collisions. This includes the AssetMetas mapping to seedling metadata and the mintingProofs mapping to proofs of asset genesis. There may be other spots where this occurs as well.

Given that all these uses happen after a batch is funded and all assets are sprouted, we should be able to use the proof.Locator struct as a replacement map key (maybe we have to use a hash of the locator).

This should allow us to support duplicate script keys. We should add a new test for that as well.