Introduce derive macro attribute processing (Take 2)

[johnhurt]

This pr includes all of the changes from @taiki-e 's pr from several years ago: #75. It looks like that pr lost traction, so I am hoping to give it new life because I would like to use the features it provides as well at the one new attribute I am introducing.

The original pr included the entire mechanism for processing derive-macro attributes, and utilizing them to allow users to

• #[valuable(skip)] - fields that should not be included
• #[valuable(transparent)] - omit types that simply wrap others
• #[valuable(rename = "new_name")] - provide a replacement name

My only contribution to this is:

• #[valuable(format = "{:#}")] - Allow for types that don't implement Valuable to included by inserting them a formatted string

I am happy to restructure or add to this pr to help it get incorporated it faster. I'm not sure why the original pr wasn't merged, but it still seems like a very useful feature. Thanks!

[taiki-e]

Thanks for the PR.

I honestly have no intention of reviewing this PR as is. The reason is that you compressed everything into one commit, which makes it very difficult to review the specific diff with my PR and requires the entire review to be done from the beginning.