Add mirror-core

[ejgallego]

Please add mirror-core and its dependencies to coq-contribs:

• https://github.com/gmalecha/coq-plugin-utils
• https://github.com/coq-ext-lib/coq-ext-lib
• https://github.com/gmalecha/mirror-core

Thanks.

[ghost]

The common properties of all coq-contribs are documented here.

Is there any hope that mirror-core can possibly conform to them?

(I haven't tried to explain the rules; in case if you think the motivation is unclear, please ask for more details)

[ejgallego]

I have moved to a different integration solution, so closing this; if anyone is interested in taking over please reopen.