Query operations

Each of the query storage struct implements the QueryStorageOps trait found in the plumbing module:

{{#include ../../../src/plumbing.rs:QueryStorageOps}}

which defines the basic operations that all queries support. The most important are these two:

• maybe changed after: Returns true if the value of the query (for the given key) may have changed since the given revision.
• Fetch: Returns the up-to-date value for the given K (or an error in the case of an "unrecovered" cycle).