Also added unit tests to for it.

This is a new implementation of a vector that operates directly on the contract storage.
It will replace the old implementation 'StorageVec' in the future.
Needs some testing first.

This allows better inspecting of the mutated value if any.
Changed for SyncCell and SyncChunk.

This accidentally did not happen, yet.

Also TypedCell::clear is now available in more contexts.

- CopyCell and CopyChunk got removed
- MutCell and MutChunk renamings

This makes SyncCell and SyncChunk more aligned to their method names.

It did not serve a purpose so far and was not used at all.
Might be reintroduced in a future commit.

CopyChunk did not serve enough purpose besides MutChunk.

Now built around `cell_at` utility.

They are going to be used by MutChunk.