Serialize and deserialize types according to their declared static type,
and add validation on the runtime types matching the declared types.

This is in contrast to previously, when we mostly only used the runtime
types to determine what to do, and harly any validation was done. The
runtime types used to be able to freely disagree with the declared
types, and we put a lot of burden on the JSII runtimes at the other
end of the wire.

Fix tests that used to exercise the API with invalid arguments.

Remove Proxy objects since they only existed to prevent accidentally
overwriting certain keys via the jsii interface, and Symbols serve
the same purpose (but simpler).

Fixes aws/aws-cdk#1981.

…nel-types