Validate more Smithy constraints at runtime

[alex-chew]

Among Smithy constraint traits, the generated .NET shims only check for the @required trait (and technically the @enum trait).

We should also add support for @range and @length at minimum.

[robin-aws]

Note I'm not adding this to https://github.com/awslabs/polymorph/milestone/1 because the dafny-codegen plugin won't be doing any validation: it will only handle wrapping an existing AWS SDK, and the service will be responsible for validation instead.

[robin-aws]

For the polymorphing use case/CLI, I'd prioritize #185 first so we at least don't silently do the wrong thing.