Fix minor consistency issues

[Neotamandua]

This PR addresses two consistency topics:

1. Every topic in rusk_abi::emit events used in Rusk is lower case and according to ZER0 should be lower case. So I changed the "TREE_LEAF" to "tree_leaf" as it was the only topic that used upper case.
2. Piecrust has an init method to run arbitrary initialization logic during deployment. For reasons explained in Change constructor to init(ializer) piecrust#381, we have changed all related naming from constructor_ to init_, this change is now also reflected in the relevant Rusk parts.