# PR 16853: Add an implementation of a Converting_merkle_tree

## Summary

Added an implementation of `Converting_merkle_tree`, a module which muxes
write requests to a `primary_ledger` to also target a `converting_ledger`.
The intention is to use this in the leadup to a hardfork, where we can run
the post-hardfork schema alongside the existing schema.

## Changes
- Add an implementation of a Converting_merkle_tree
- Added tests using a temporary `Account.Unstable` type. Subsequent work
  will change this to a type which is more reflective of the upcoming changes
  to the `Account` type.