[State] Replace IAVL

[silasdavis]

IAVL lacks a maintainer and has fundamental performance issues (see
cosmos/cosmos-sdk#829 and cosmos/cosmos-sdk#7100)

The Cosmos SDK team are planning to replace their use of IAVL with https://github.com/lazyledger/smt but need to make breaking changes to KVStore, namely SMT does not support iteration over keys

https://github.com/silasdavis/trieste could still be an option, but needs work. If we want to have a defined 'file format' storage scheme (based on something like capnproto/flatbuffers) then it might be useful to own this layer.