`anyhow::Result` shouldn't be used for libraries

As per https://github.com/oxidecomputer/meta/blob/master/engineering/rust-crate-zoo.adoc#anyhow:

The anyhow crate provides useful error handling for applications. The main features are:

anyhow::Result<T> as a general return type, with the neccesary impls to be able to "`?` all the things."

A .context method to help troubleshooting later.

This lines up with application use-cases because in applications, you are often collecting errors from your dependencies, and need to combine and/or propogate them upwards. You often are not writing your own bespoke error types.

Anyhow requires an allocator, and therefore, is not appropriate for most of Oxide’s no_std code.

If you’re writing a library, see thiserror instead.

and https://github.com/oxidecomputer/meta/blob/master/engineering/rust-crate-zoo.adoc#thiserror:

The thiserror crate provides a nice derive for the Error trait. This makes it very useful when writing a library, as you’ll probably be crafting specific errors to return to your callers.

We need to switch upstairs code to return the standard Result instead of anyhow::Result.

Start by locating the “upstairs” code and all uses of anyhow::Result in the repository, then review the referenced Rust crate guidance. The work is done when the relevant library code returns standard Result types instead of anyhow::Result, with existing behavior preserved.