In-memory trie operations (#1408)

* small cleanups in trie

* Memory trie skeleton

* decode nodes from RLP

* memorytrie -> memorytriedb

* implement Trie for MemoryTrie

* partially implement insert

* implement trie insertion

* don't load whole trie into memory, lookup across memory and db

* re-implement insertion and lazily load necessary nodes from DB

* node removal w/o fixing

* reduce churn in node storage

* finish remove implementation

* committing the in-memory trie

* reload root node after commit

* generate elastic arrays when converting nodes to rlp

* port triedbmut tests over to memorytriedb, fix a few bugs.

* hash count and dirty flag

* initial attempt for node inspection

* back to skeleton

* inspection framework

* implement insertion

* no panic paths in insert

* implement deletion without fixing

* node fixing

* commit nodes to db

* tracing targets and bugfix

* get tests to pass with a lot of tracing

* set playpen iterations to 10

* remove some tracing statements

* make TrieMut::root take &mut self

* replace triedbmut with memorytriedb

* treat empty insert value as removal

* add test for null insert

* fix some style concerns

* trie: use nodehandle for root_node, minor cleanup