Explore index and index_assign for StorageMap and other dynamic storage types

[mohammadfawaz]

#4331 and #4334 attempt to introduce index and index_assign to StoargeMap and StorageVec respectively. These methods are then called by the compiler when [] is used to support syntax like storage.map[a] = b.

As we're not sure how to go about supporting [] for arbitrary types just yet, I've closed those PRs. They do contain important code though for future reference, particularly code that de-sugars [] to calls to index or index_assign.