Add immutable style overriding of constants

[PatrickAlphaC]

Would love to see this in the docs somewhere.

https://vexil.notion.site/Immutables-in-Huff-7db5cb6436494fa195e73c43e989b17d

Just so that if someone looks for immutable in the docs search bar, this comes up.

[refcell]

+1 - let's port this notion page to the docs to describe the implemented tradeoff