The implementation of substrate-rpc-custom was a bit verbose and repetitive.

This commit makes the implementation simpler by intruducing a struct which
represents query for a typed value in storage. The new struct is called
StorageQuery. A StorageQuery wraps a raw StorageKey but is not directy
constructable. To construct a StorageQuery, the user must supply an
implementation of a srml_support::storage::generator trait such as
StorageValue or StorageMap.

A type implementing one of the generator traits can be aquired by:
A) marking a storage item as pub within a call to decl_storage (recommended)
or B) implementing one of the generator types manually.

While option B may sometimes me necessary, it's not recommended because
separate manual implementaions may lose sync with the original definition.