Typescript declaration

[john-osullivan]

Building this in did require adding ethereum-types as a dependency, let me know if y'all would prefer I send this content over to the DefinitelyTyped community-made types. I wanted to try making this PR directly to the source repo, though. Directly incorporating these types makes it less likely that they'll become out of date, and I think this is a good demonstration that the Typescript overhead is actually quite small.

I was originally reluctant to add it into my dev flow, but the Intellisense improvements are just too good to pass up.

The forced pushes here are just me squashing commits I made while iterating.