**Motivation**

Being able to create a `Trie` struct representing the world state based
on the engine's internal value store (reading from a specific table in
the case of libmdbx or map in the case of in_memory). The created trie
would also use the given block's state root as its root in order to
query historical data build new blocks based on the previous block's
state, and easily process re-orgs

<!-- Why does this pull request exist? What are its goals? -->

**Description**
Added features:
* Add `world_state` method to `StoreEngine` api, which builds a `Trie`
from a block number and the engine's internal database (or value store)

Code refactors:
* Remove `DB` generic from `Trie` and instead store `Box<dyn TrieDB>`
(This is because the generic wouldn't allow us to return a single
generic `Trie<DB>` from different `StoreEngine` implementations)
* Store `Arc<Database>` instead of `&'a Database` in libmdbx impl of
`TrieDB` (This is so we can box it)
* Store `Arc<Database>` instead of `Database` in
`engine::libmdbx::Store` (Complies with above change)

Misc:

* Add `Trie::new_temp` test-feature gated method to simplify trie
creation in tests

<!-- A clear and concise general description of the changes this PR
introduces -->

<!-- Link to issues: Resolves #111, Resolves #222 -->

Closes None, but is needed for #391

---------

Co-authored-by: Tomás Arjovsky <t.arjovsky@gmail.com>