The `jsii` compiler incorrectly mapped types homonym with a standard
type (`String`, `Number`, ...) to the primitive type on usage sites.
This commit corrects this behavior and adjusts the C# tests accordingly.

Additionally, when the return type of a callback was an interface, the
C# runtime was unable to determine the correct JSII type to use, despite
it has the information on the type of the callback method available. The
resolution behavior has also been fixes, as well as a couple of other
glitches that became apparent as a result of the `Number` type starting
to be correctly represented as the `@scope/jsii-calc-lib.Number` type.

Fixes #290
Fixes aws/aws-cdk#1027