Drop thiserror from crates

[oscartbeaumont]

I think it's worth just manually implementing them to avoid the macros and extra dependencies.

It's a great crate and the impls we want will be the same as it generates. We are just a library and the idea is to get Specta as early in the build process as possible.

This is especially true when we have a macros feature which wouldn't technically disable macros if we stick with using thiserror.