Nimony: ref T and ptr T support plus nil

[Araq]

• needs a .compilerproc mechanism and allocator support
• support nil for these - tested by add test for nil pointers #319 at least
• support (ref T)(fields: values...) construction - accepted in frontend
• lower ref object construction (NewOconstrX)
• derefs for ref object dot fields
• make deref operator [] part of system.nim: a .magic for pointers/refs
• nominal ref/ptr objects - possibly as own type kinds

[Araq]

@metagn Work on the frontend aspects of this please.