re-thinking: track the source of settings

[dnaka91]

For debugging purposes (mostly on the user end), it would be great to be able to keep track of what source a setting came from. Especially when the same setting can come from the several sources, it can be a great way to figure out where a setting comes from.

For example, let's say a CLI app has some default settings, a global settings file, another one for the local project it works on, environment variables and eventually clap as last layer for settings input. That brings 5 different locations for potentially the same settings to the table.

When there are many settings, it's sometimes unclear why a program behaves a certain way, and then it can help to find out where some specific setting comes from.

Probably an extension of #323 and #324.

[martinvonz]

How does this issue related to Value::origin and to #319?