Implement syntax-rules macro system.

[CinchBlue]

I cannot implement syntax-case. But I can do syntax-rules macros!

[CinchBlue]

Syntax-rules macros are being implemented without syntax objects, it appears. Instead, we are using a MacroContext to keep track of lexical information and keep track of sets of scopes so that the information for macros need only exist at macro-expansion time. This may need to change later once we start implementing debugging functionality, but we can consider this later.