Add a feature to use Display as Decimal's Debug output

[shuoli84]

As a user, the default Debug impl is hard to reason, so I add a feature to let client switch to human friendlier debug output.

[neogenie]

Thank you for the feedback and participation in the project!

I don't think it's worth using the features’ mechanism for these purposes, as it will create issues during use of --all-features.

The first thing that comes to mind is to use compile-time variables, where some display parameters are already configured.

Also, for these purposes, we can use Debug (:?) and Alternate Debug (:#?), for example, in combination with a compile-time flag (when the flag is set, one of the above-mentioned placeholders replaces Debug output with Display).

[shuoli84]

Yes, you are right. the feature flag is not pure additive, which makes it tricky.