Rename Transform struct to TemplateTransform

[xrn]

Issue #, if available:

Description of changes:
I need to use in my stack implementation Fn::Transform (https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-transform.html) but this intrinsic function is not implemented. I would like to implement it as cloudformation.Transform() but this name is already blocked by Template.Transform struct name. I would like to rename it and then in the next PR implement cloudformation.Transform

This PR is also solving #294

If you have any ideas on how this can be implemented better - by for example creating a template package or sth like that I am happy to work on this.

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[xrn]

@PaulMaddox - what do you think?

[PaulMaddox]

I'd rather not make such a breaking change to the public API.

It's probably easier to name the intrinsic method something like cloudformation.TransformFn(). I know that's a bit ugly as it differs from the others.

[PaulMaddox]

I believe this can be closed now that #352 is merged.