Make error.rs its own crate

[kevinheavey]

I often find myself wanting the code in error.rs without needing the rest of the anchor_lang crate. It would be nice if there was an anchor-error crate to that end

[acheroncrypto]

The majority of dependencies of anchor-lang comes from solana-program and borsh crates, both of them seem to be imported from the error.rs file so I'm not sure about the benefits of splitting this into its own crate. What's the benefit of this other than a slight improvement of the compilation experience?

[kevinheavey]

If I could get Labs to split up solana-program, the anchor_lang build time would look much more significant 🙃