Handle structs (except with complex data, that's nasty)

[samcunliffe]

This PR is a single git cherry-pick of the merge commit of #278 but now targeting main.

There are a few ways we could tackle this (including rebasing #250). I think one fresh PR per merge commit is the sanest and my favourite way. There are 4 PR merge commits to the pairdev branch so I'll open 4 PRs which are cherry-picked clones of your originals.

Part of

• 181

Continues from

1. 304

2. 310

Supercedes

• 193

• 250

Actual code notes and change description

... are in

• 278

@willGraham01 please double check the cherry-pick worked and make a final final pass review.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (afb3596) 25% compared to head (f69f59a) 25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #311 +/- ## =================================== Coverage 25% 25% =================================== Files 63 63 Lines 3596 3596 =================================== Hits 903 903 Misses 2693 2693 ```

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