Algebraic data types and structural pattern matching RFC

[elcritch]

I believe some of the work @ElegantBeef has done with 
[fungus](https://github.com/beef331/fungus/tree/commonfields) is useful to 
consider. In particular, how to handle var vs non-var matches? Also, having a 
conical if/else idiom would be useful. There's some ways to one line if/else 
matches but they're all kinda hacky.