Unify changeset APIs between all structures

[LLFourn]

• Each structure has a different name for its changeset. For example TxGraph it's called Additions while IndexedTxGraph is called IndexedAdditions. Rather they should have the same name and users should just use different module prefixes e.g. tx_graph::Additions or indexed_tx_graph::Additions.
• Since these are monotone they don't need a "preview" methods. You can always just apply the update right away. I'd also include LocalChain in this policy despite it not being monotone (yet).

[evanlinjin]

Since we are already on this, can I request for some more renamings?

• CanonicalTx::observed_as should be renamed to CanonicalTx::chain_position.
• CanonicalTx::node should be renamed to CanonicalTx::tx_node.

I think this will provide more clarity.

Thanks @danielabrozzoni