provide a mechanism to pass references to keywords

[ldalessa]

I don't currently see a great way to pass a reference using a named keyword. When I use a reference typed keyword I get compilation errors, and if I use an untyped keyword and use std::ref it just copies the ref without decay. I feel like I should be able to use a reference typed keyword successfully, and I also feel like there is a small argument for implicit decay for reference_wrappers in both typed and untyped settings (like tuple).