Move anyhow integration behind a feature flag

[sonro]

BREAKING CHANGE

Hide Error::downcast and Error::from_anyhow behind an "anyhow" feature flag. These are currently the only public APIs using anyhow types. Moreover only publicly re-export anyhow crate behind the feature flag.

This allows the default API to keep the implementation separate from the user and the "anyhow" feature-set to allow conversion between the two types.

[sonro]

ExitCode implmentation for anyhow::Error should only be public behind the flag.

[sonro]

Due to the way the error_from macro invokes anyhow's macros, anyhow must remain public. Therefore we will not use a feature flag to gate off the anyhow integration.