refactor: Default more functions to using open_extensions

[croyzor]

A combined version of #616 #617 and #618

[acl-cqc]

This looks pretty good, but I've been prodding around here myself too and discovering how much of a maze these various methods are :-(. Does this make add_op_with_parent also use open_extensions?

[acl-cqc]

Does this make add_op_with_parent also use open_extensions?

No, it doesn't :(. So I think it's non-obvious, indeed counterintuitive, that which add_op/add_node function you use, changes what Option<ExtensionSet> you get. I think we should probably focus on figuring out why using open_extensions everywhere doesn't work.

[croyzor]

add_op_with_parent still uses pure, or there'd be no reason to change the implementation of any_block_builder.
Still need to investigate what needs done to update add_op_with_parent, unfortunately

[acl-cqc]

Superceded by #622