Split up into separate pakages

[gregwebs]

Overview

This is a re-structuring. The functionality remains the same.

• a base stackfmt package. This package is just for stack traces and does not do anything with errors.
• A separate slogerr package for those looking to add structured errors.
• An errwrap package for wrapping and unwrapping errors
• Maintain the existing errors package. This is designed as a replacement for the existing errors package that introduces wrapping with stack traces

The errors package will re-export some items from other packages.
However, it is exporting much less overall. I think the previous package had too large of an API surface, making it difficult to navigate from looking at the go docs.