feat: deprecate "cloudformation" namespace in favor of "CfnXxx"

[eladb]

Rename generated CloudFormation resource constructs from cloudformation.XxxResource to CfnXxx. This fixes #878 and eliminates the use of namespaces in the CDK.

This is done in a backwards compatible way, which means that we still generate the old resources under the cloudformation namespace so we can remove them in a subsequent release. Those resources also include a deprecation warning which is emitted upon cdk synth.

Documentation updated to reflect changes.

Related: aws/jsii#283 and aws/jsii#270

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[RomainMuller]

That looks... maybe less dapper than I had imagined, but not too bad either.

[rix0rrr]

This is fine.

[skinny85]

I like it. I think CfnBucket conveys L1-ness better than cloudformation.BucketResource did. 👍🏻

[mergify]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

We will shortly assign someone to review this pull request and help get it
merged. In the meantime, please take a minute to make sure you follow this
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