performance: perform dataset operations directly on index

[jeswr]

There is lots of room for optimisation in methods such as union and difference; in particular, whenever other is also an N3Store and they share an index we can just merge / diff / compare the indices for set operations.

See #392

[jeswr]

Many functions would benefit from using #structuredClone in their implementation. However, this would require an mver bump to deprecate old versions of Node as #structuredClone is only available from Node v17

[RubenVerborgh]

And browser? Maybe good to use it conditionally for now.

[jeswr]

Seems to be supported across all browsers now https://developer.mozilla.org/en-US/docs/Web/API/structuredClone#browser_compatibility