Make `Digest` support `StorageAppend` (#5922)

* Make `Digest` support `StorageAppend`

This adds support for `StorageAppend` to `Digest`. Digest is just a
wrapper around a `Vec` and we abuse the fact that SCALE does not puts
any special marker into the encoding for structs. So, we can just append
to the encoded Digest. A test is added that ensures, if the `Digest`
format ever changes, we remove this optimization.

* Update weight

* Update frame/support/src/storage/mod.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/system/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>