refactor(pkg/trie): Encapsulate in-memory trie impl in its own package

[dimartiro]

Changes

This PR is to create an independent inmemory package within pkg/trie to encapsulate the InmemoryTrie implementation there.
This is useful to prevent mixing the new code for the lazy loading trie implementation (aka: TrieDB) with the old inmemory trie implementation

Summary of changes

• Move all files related with the in-memory trie implementation to /pkg/trie/inmemory folder
  • The structures or methods that are shared between different trie implementation remains in `/pkg/trie/
  • Update imports to import inmemory_trie as alias instead of using old trie package
  • Add new layout_test as part of /pkg/trie/inmemory to test the layout for the in-memory trie implementation
  • Move /pkg/trie/proof to /pkg/trie/inmemory/proof since it's coupled with the trie implementation
• Add a Trie parameter to layout.Root function to make it generic based on Trie implementation

Tests

make test

I also did run a node locally from genesis until block 1M (stopping it and restarting it a few times)

Issues

closes #3804

Primary Reviewer

@jimjbrettj

[jimjbrettj]

lgtm