Improve APIs for Tries in Runtime

[shawntabrizi]

This is a refactor and improvement from: #3881

• sp_runtime::proving_trie now exposes a BasicProvingTrie for both base2 and base16.
• APIs for base16 are more focused on single value proofs, also aligning their APIs with the base2 trie
• A ProvingTrie trait is included which wraps both the base2 and base16 trie, and exposes all APIs needed for an end to end scenario.
  • This is important because when writing benchmarks, you need abstractions to be able to create and prove tries of different sizes.