use const field instead of internal RefValue

[KristofferC]

Better for performance, avoids using internals and avoids awkward [].

[tecosaur]

I've taken a bit with this, since it didn't seem like such a "clear win" to me as the other PRs, but why not 🙂

Aside: how on earth did this branch end up with this particular name? 😆

[KristofferC]

I've taken a bit with this, since it didn't seem like such a "clear win" to me as the other PRs, but why not

To me, it is a clear win because of:

• Clarity: We want the ability to mutate some fields but not others. Julia has a feature for this, const-fields. Going through the hoops of wrapping something in a container doesn't do anything better.
• Legibility: Instead of having to use [] everywhere these are just used like any other field.
• Performance: Instead of allocating one mutable wrapper for each field this can allocate a single one where all the mutable fields are stored.