Add checks for type declarations (#2448)

* Add overloads for Matrix add and subtract

* Add check for types/index.ts

* Fix type errors in types/index.ts

* Fix a couple execution errors

* Run test:types as part of test:all

* Fix remaining errors

* Replace types/index.ts comments with asserts

* Add tests for narrowed type inference

* Add dual-purpose comment at top of types/index.ts

* Update AUTHORS

* Use Glen's alternate test:types suggestion

Author: samestep

AUTHORS

@@ -181,6 +181,7 @@ Glen Whitney <[email protected]>
 Divya Yeruva <[email protected]>
 Yeruva <[email protected]>
 Eternal-Rise <[email protected]>
+Sam Estep <[email protected]>
 yifanwww <[email protected]>
 NattapongSiri <[email protected]>
 Hjortur Jonasson <[email protected]>