Ghoul - an undead themed lisp interpreter

Ghoul is a lisp interpreter that aims to be simple to understand while being a bit more advanced than a naive interpreter.

Explicit goals:

• Easy to understand code base
• Support simple integration of Golang code
• Proper tail call optimizations
• Hygenic macro support

Non-goals:

• Fast code execution - It should be easy enough to drop down to golang code if something needs speed.
• Comprehensive standard library implementation written in Ghoul - it should rely on Golang implementations as much as possible.
• Special handling of datastructures in the interpreter - special syntax for e.g. maps should be handled by macromancy!

Notes:

TODO: Add a Foreign expression type that can be used for arbitrary structs
TODO: Write "wraith" - tool for wrapping Go code for Ghoul use
TODO: Wrap Golang standard library so it is callable from ghoul
TODO: Implement module system so that code can be included when required (as opposed to including it all upfront).
TODO: Make error messages contain line and column of failed expression. Derived expressions should as far as possible point to their original version.
TODO: Use fn instead of lambda?
TODO: Use do instead of begin?
TODO: Use def instead of define?
TODO: Implement macros in the macromancy pakage!
TODO: Make macro elipsis associate to preceeding identifier so more complex code patterns can be expanded.
TODO: Add keyword literals to macro matching TODO: Implement support for multiple elipsis in macro matching and in expansion bodies.
TODO: Ensure macros propagate source location to expanded code
TODO: Implement pathological macros (like in racket macro docs 16.1), e.g. (swap tmp other).
TODO: Clean up tests into separate files with distinct areas
TODO: Implement an error printer
TODO: Make Pair struct private and replace usages with a Cons(Expr, Expr) function returning a *pair.
TODO: Make List.Tail() return (List, bool) and move usages of tail() to that
TODO: Make Pair interface have a First() Expr and Second() Expr method.
TODO: Replace cond with match keyword and have it use pattern matching.
TODO: Implement a symbol table and use integers instead of strings to compare/find the right values
TODO: Add logging (to lowest log level) essentially everywhere and make sure it is disabled in ghoul command unless some param is given.