[State] Expose account storage hash

[silasdavis]

When moving to the merkle forest one of the aims was to reintroduce the ability to provide a single-account storage hash (rather than only being able to provide the total state root hash). We currently have individual storage trees per account (we do need to understand performance implications of this better though there is a benchmark). It would be nice to expose this knowledge of the info/query RPCs. It allows you to do things like watch for changes to an account.