Add hygienic macros

[weskerfoot]

I really think possum v2 could benefit from a hygienic macro implementation. Streams could be implemented in terms of this, as well as many other things which would normally be special forms.

[darkf]

Streams in possumv2 are hard-coded wrappers around .NET streams. In possum-ocaml, I would like to rectify this by including a stream/iterator based abstraction, but how to do this is still up for debate. I may use functions inside records to sort of duck-type the system, but I am not sure how clean this is. Relevant issues are here and here.