Add commands to create compiled models

[AndriySvyryd]

Warn when using and old model
Enable NRT-enabled code generation
Remove the need to call FinalizeModel before UseModel
Split out the boilerplate part of the compiled model

Fixes #1906

[AndriySvyryd]

Ping. We branch for preview5 tomorrow.

[smitpatel]

Reviewed 3rd commit here which has title "Add commands to create compiled..."

[AndriySvyryd]

@smitpatel Much appreciated. Forgot to rebase.

[ghost]

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[ghost]

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[bricelam]