Represented textual expression using CustomDebugStringConvertible.

[es-kumagai]

Represented textual expression using CustomDebugStringConvertible protocol instead of CustomStringConvertible protocol.

Motivation

This motivation is that I'd like to customize a text representation of DecodeError instance to displaying error message in my application. I tried to make DecodeError conform to CustomStringConvertible protocol, but it was already conformed to it, so I couldn't do that.

Express as a debug information

In addition, I think the textual expression of DecodeError that are provided by CustomStringConvertible seems to be debug information.

In this reason, I'd like to propose to express the information using CustomDebugStringConvertible protocol instead of CustomStringConvertible protocol.

For the same reason, I'd like to propose that to Extractor and KeyPath too.

[ikesyo]

Sorry for the really late response 🙇

But this makes sense! Thanks for the pull request 👍