Make #instance method name configurable

[janlelis]

Unfortunately, Object#instance clashes with some other libraries. For example, in does not work with more recent Rails versions (see https://github.com/rails/rails/blob/v5.0.2/railties/lib/rails/application.rb#L93-L95). Another example is stdlib's singleton library: https://github.com/ruby/ruby/blob/trunk/lib/singleton.rb

Another solution would be to rename it to something that is less likely to interfere with other libraries. Or maybe even go the refinements route?