Expose mixin locations

[Morriar]

Summary

• Add Mixin#location to the Ruby API.
• Track mixin offsets in Rust so the location covers the full include / prepend / extend expression, not just the referenced constant.
• Add Ruby API signatures and coverage for include, prepend, and extend locations.

[vinistock]

As discussed on Slack, I think we don't need to pay the memory price to remember the full mixin's location. Especially considering that someone can do multiple operations in a single line, which diminishes the relevance of the mixin location vs the constant reference location.

For example:

class Foo
  include Bar, Baz, Qux
end