Investigate separate set of exports for tests

[mijamo911]

Currently Ion.ts exports a bunch of classes just for test visibility. We may want to look into having two sets of exports, one for consumers and one for tests so that we're not leaking too many implementation details

[mijamo911]

Addressed by #28

[therapon]

Things look better but I think there is more to be done here. So leaving this issue as open

[zslayton]

This is mostly fixed, with remaining visibility issues being tracked separately.