Collision detection

[yisraelU]

Draft to address collisions with stdlib types and keywords
keyword protection is done with underscore as using backticks (which would be idiomatic scala) creates a new problem where we append to names "Gen" or "Op" etc.. a backtick must wrap the entire identifier
I found this to be sufficient reason to avoid switching to backticks as it would involve significant code shuffling .
This draft is to be reviewed for correctness . I will be making further commits to address code conciseness and quality

[Baccata]

Ain't got time to review proper before next week, but, I think the most desirable solution would be to remove the underscore-logic altogether in favour of a local rename.

Say you have the following in a smithy model

namespace foo 

@uniqueItems 
list Set {
    member: String
} 

then you'll have foo.Set colliding with scala.Set. When handling such things manually, what we'd do is :

import scala.{Set => SSet} 

Doing this would be facilitated by the fact that we're retaining structural information for longer, meaning we can transform the NameRefs everywhere they need to be before collapsing the structure into text.

[yisraelU]

@Baccata I'm not using underscore for collision renaming.
It's only being used for keyword renaming i.e. a val name type

[yisraelU]

this will fix #255 too