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* Staticcheck fixes

This fixes all staticcheck warnings for this library.

CloseNotifier has been deprecated for a while.

Also, ditch the "environment" context. We don't actually _need_ this.

Only use custom encoders for values.

fixes #65

Also, remove SetEncoder. Always derive the correct encoder from the request.

We *always* expect environments to be able to return a context so we might as
well enforce this with the type system.

These aren't placeholder contexts.