Support Smithy 2.0 features

[alex-chew]

Smithy 2.0 is now available. Polymorph is built on Smithy 1.0 and should be updated.

There is a migration guide for upgrading from version 1.0 of the Smithy IDL to version 2.0: https://smithy.io/2.0/guides/migrating-idl-1-to-2.html

Most of the changes seem to only affect the IDL, and not the semantic model, but there are (at least) these notable exceptions:

• The @box trait is removed, and primitive root-level shapes have no implicit default
• The set shape is deprecated in favor of a list shape with the @uniqueItems trait

[robin-aws]

I assume it will be far easier to ONLY support Smithy 2.0, and require updating existing Smithy 1.0 models? I'd be curious to see whether the other smithy-*** packages manage to support both at once.

The set shape is deprecated in favor of a list shape with the @uniqueItems trait

This one is interesting, will definitely be a breaking change for any model that uses set, as I assume that becomes a Dafny set<T>.

[robin-aws]

Qualified the title of this a bit: since upgrading to newer versions of core Smithy packages we can handle 2.0 models just fine, and the remaining work is to handle new features like @enum. But I think it's more important to first cleanly reject those features and other unsupported ones via #185.