fix #18 export Mutators type declaration

[bentorkington]

No description provided.

[maetl]

We need to export usable TypeScript definitions from this lib at some point. I have gone through this process with the Rung lib on NPM and it seemed to work okay without porting the entire project to TypeScript.

I also suspect that some of the older dynamic programming stuff here (like this example of the mutator mixin) might need to change a bit. This library was very much designed in my head as an ideal modular trait-based API, but in the conversion to JS it ended up somewhere a bit different than what I wanted.

[maetl]

v0.12 on NPM has these changes incorporated into the types exports. As I said, I’m not 100% sure how usable these are and it would probably be good to deal with the issues identified in #17 to get this all sorted before adding any new features here.