[Feature Request] Autopromotion of literal types to concrete types

[Mogball]

There are a large handful of issues that fall into the same category, so I am combining them into a super-issue to make tracking easier.

The core problem is that we need to "autopromote" literal types to concrete types in certain situations.

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1. For example, string literals should be autopromoted to String when creating a variable from StringLiteral: [BUG] StringLiteral concat works only in narrow cases #9
2. List literals cannot be iterated or indexed, because they should be promoted to some dynamic list type: [BUG] for i in [1,2,3] is broken: ListLiteral ... does not implement the '__iter__' method #88, [BUG] Cannot access list element by index #89

[willghatch]

At this point we have proper IntLiteral and FloatLiteral. The nonmaterializable feature is ready for similar behaviors by things like StringLiteral and ListLiteral. But I don't think those necessarily need compiler-specific support -- I think it can all be implemented now in the stdlib. I tried making our current StringLiteral be @nonmaterializable(String), and it gave a lot of errors where StringLiteral is used dynamically in the stdlib. So I think this can be handed off to the stdlib team now. Thoughts @JoeLoser @Mogball ?

[willghatch]

Well, this specific issue has all of its sub-issues completed, so I'm going to close this one. I think we should make issues to make these other literal types be nonmaterializable and convert to their dynamic counterpart, but I still think that that can move to stdlib now.