Result: map_err and deconstruct

[nz]

Surprised I went this long without Result#map_err, I really need to make a full checklist of Result methods and see what else may be missing.

Also deconstruct is pretty useful for pattern-matching on Result. Because Ruby has Exceptions, but exception-handling is optional, I find it fits the job that Errgonomic is doing to rescue certain (domain-applicable) exceptions, and return them with the Error side of a Result. Using deconstruct and Ruby pattern matching gives me a reasonable approximation of matching an enum in Rust.

result = Errgonomic::Result::Err.new(StandardError.new("nope"))
case result
in Errgonomic::Result::Ok, value
  "Measurement is #{value}"
in Errgonomic::Result::Err, String => msg
  "Measurement failed with a message: #{msg}"
in Errgonomic::Result::Err, Exception => e
  "Measurement produced an exception -- #{e.class}: #{e}"
end # => "Measurement produced an exception -- StandardError: nope"

So far I think this is more helpful than it is uncanny valley. But this whole gem kind of flirts with that space.