Add page on optimization to user guide

[tomwhite]

See #381.

This relies on #396 to make it easier to cross-refer operation IDs in the debug output to the visualizations.

[tomwhite]

This is very helpful for me to understand what optimization options we have, but for a user this would seem like an unmotivated exposure of pretty deep internals. Is this intended as developer docs?

I wrote it to capture what I'd implemented so far on the optimization work, but you're right that it's not pitched well for users!

I hope that we can make the more aggressive optimizations enabled by default in which case users won't need to care about these settings, although there will likely always be cases where an advanced user wants to change things. But we're not ready to do that quite yet.

Might be worth leaving this open until we have more experience with optimizing some example workloads.

[TomNicholas]

To be clear - as a temporary record of the state of possible optimizations, this is great. My criticisms are more looking towards what would be most useful for users in the future.

[tomwhite]

To be clear - as a temporary record of the state of possible optimizations, this is great. My criticisms are more looking towards what would be most useful for users in the future.

It's very helpful feedback - thanks!

[tomwhite]

I've re-worked this page to reflect the new defaults (multiple-input fusion is enabled by default now), and also to move the advanced settings to the end since they should be rarely needed. I think this is generally useful for users should should be Ok to go in now @TomNicholas.