state_getSize RPC for storage maps (#6847)

* Fancy compact encode/decode impl for compact solution

* Make it optional

* Remove extra file

* Update primitives/npos-elections/compact/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Final fixes.

* getSize rpc should work for maps as well

* Fix future types

* Remove minimum_validator_count stale const

* Update client/rpc/src/state/mod.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* "Optimize" `storage_size`

* Remove unused import

* Update doc

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>