What this PR does / why we need it?

• I want to add methods to MPTTrie<DB> in another crate.

  Limit by the orphan rule, I have to use a trait to add methods to MPTTrie<DB>.

  • If I use a trait to add methods to MPTTrie<DB>, I could NOT write the following code:

    pub trait TrieExt<BD>: MPTTrie<DB> {
        /* Use methods of `MPTTrie<DB>` in trait body. */
    }

    , since MPTTrie<DB> is a struct, but I can write the following code:

    pub trait TrieExt: Trie {
        /* Use methods of `Trie` in trait body. */
    }

• Some methods of MPTTrie<DB> are not exposed, for example, get_proof(..) and verify_proof(..).

• Another reason:

  In fact, MPTTrie<DB> is a Trie, we should use it as Trie directly.

  Trie is in a higher level of TrieDB in abstract.

  Treat MPTTrie<DB> as as a TrieDB is not reasonable.

What is the impact of this PR?

No Breaking Change