Kernel generated by FE should have strong-mode type checks

[sigmundch]

(this is not a new request, but I realized that we didn't have a tracking bug for it)

/cc @kmillikin @stefantsov

[sigmundch]

as discussed in our last sync up, our plan is to take the existing type inference implementation in the front-end, and include type checking and insertion of type checks at that point.

[stereotype441]

As of 9aa3a0e I believe the front end inserts all the necessary runtime checks and forwarding stubs due to generic covariance. I'm just beginning to work on adding runtime checks due to implicit downcasts.

[stereotype441]

As of 327adac I believe the front end inserts all the necessary runtime checks due to implicit downcasts. Runtime check insertion is now complete.