Use blanket impls to add convenience methods to all errors

[sruggier]

This defines two traits, each defining a convenience method, as_inline_error_chain and as_array_error_chain respectively, as a more convenient way to construct InlineErrorChain and ArrayErrorChain adapters.

Finally, it also defines corresponding blanket implementations for each one, so they automatically apply to any value whose type implements std::error::Error. This makes it possible to use err.as_inline_error_chain() instead of InlineErrorChain::new(&err), and err.as_array_error_chain() instead of
ArrayErrorChain::new(&err).

[sruggier]

As discussed in oxidecomputer/omicron#9923. After playing with the idea a bit in a minimal test crate, I decided to go ahead and implement it. I made it return the adapter rather than producing a string directly, so that, true to the crate name, it would also be applicable in calls to slog macros.

[sruggier]

To be clear, I don't see this as mutually exclusive with adding an extra string formatting method on the derive macros, I just happen to have tried this first.