Challenge: write a reference counted slice that works as much as possible like a built-in slice

[rikki cattermole]

I think that we just came to an agreement that the metadata stuff should 
be handled separately potentially using different memory.

I suggested this as an escape hatch with my custom reference type back 
in my second comment!

https://forum.dlang.org/post/smc9r2$7nh$1@digitalmars.com