u3d/asset: add feature that enables easy inspection of asset in a Unity project

[niezbop]

Pull Request Checklist

• My pull request has been rebased on master
• I ran bundle exec rspec to make sure that my PR didn't break any test
• I ran bundle exec rubocop to make sure that my PR is inline with our code style
• I have read the code of conduct

Pull Request Description

This is some work I have done recently to look for asset usage in a Unity project, and I believe it belongs in u3d. It enables one to easily browse the asset in his project and find how they reference one another, either by guid or by name.

Sample command possible:

$ u3d console
Welcome to u3d interactive!
>> U3d::Asset.glob('Assets/Some/Path/*').map {|asset| asset.users}.flatten.uniq
[...]

Which is quite useful in my opinion and makes for a powerful debugging tool.