Documentation: Policy for borrowing Copy structs

[carlaKC]

This is very low priority, I just found it interesting 🦀

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In sim-ln we borrow &PublicKey when we pass it to functions, rather than copying it (PublicKey).
By contrast, LDK mostly just copies around PublicKey.

I was interested in when we should borrow vs copy smaller structs, and did some digging:

• Seems like it's more efficient to just copy the value when it'll fit into a register (64 bytes)
• Apparently the compiler will optimize-away borrows on small structs anyway (or pass by reference for large structs)
• There isn't much difference when you benchmark it (in the scenario that article tested)

All of my sources here are reddit comments (😅 ) so it would be interesting to confirm:

• That the compiler will optimize-away borrows of small structs
• That the compiler will under the hood borrow large structs rather than copying them

We're not going to get some big performance gain here, so this really is more an intellectual exercise / matter of standardizing how we do things. Seems reasonable to update docs to advise:

• If < 64 bytes -> Copy
• If > 64 bytes -> &borrow

This way we're covered even if the compiler isn't being super smart (on my to-do list to figure out if it is).