Pre-RFC: Custom Literals via Traits

[newpavlov]

@mcy

The marker type is convienient because it gives a unique point of reference to the trait system for the desugaring, and places the literal into the module system. I think not having a trivial desugaring, like with every other operator, is a bad idea.

Same argument can be made for macro based system (which was roughly described here), which will have trivial desugaring as well.