Draft proposal for mixin template intermediary storage

[Hipreme]

https://github.com/dlang/DIPs/pull/237/files?short_path=8527ec5#diff-8527ec51661ec30739a45e52edbb0f5299a3aad7b8eafbeb21d3b09d7c42ad82


I have been using `mixin template` for quite some time as I was a 
bit bothered with its usability and how dependent of other 
features it actually is. I'm proposing a more readable, self 
contained and faster mixin template. I want to know some of your 
thoughts