bettersum (v0.7.0): mixin-based sumtypes

[DanielG]

Dub package:
https://code.dlang.org/packages/bettersum

GitHub repo:
https://github.com/dewf/bettersum

Check the README for usage.

I realized a few weeks back that I could probably get 90% of what 
I want (minus legit pattern-matching) from sumtypes just by 
writing a mixin. So, here we are.

Feel free to suggest ideas for even more concise or useful 
pattern-matching-like idioms. I did what I could with what I 
know, which admittedly ain't much :)

I need to return to a more important project for awhile, but I 
hope to get this to a v1.0 release by the end of the year.

Note: the first thing I want to fix in the near future, is the 
error reporting (for parsing errors) - it will be off by a token 
length, column-wise - and is very likely broken in other ways. 
Plus I haven't tested the parser much, so don't expect it to be 
robust against weirdness.

Also I need to update the example to show how to use snake_case 
instead (ew).

We'll get there!