typechecker: remove continuation typedef

Summary: It's actually a (deprecated) abstract class name in the runtime,
 which comes into play when interpreting whether the expression
 `instanceof Continuation` is OK. Since `Generator` is the name going
 forward, let's just use that.

Reviewed By: @jwatzman

Differential Revision: D1650100

Signature: t1:1650100:1414797172:226254a8831cd424a705e6e04f9b6c32d6833bdf