Removed AST::Fn, no longer used.
Added initial setup for Value::Fn with non-fixed args.
Added creation of vararg lambdas.
Added creation of lambdas with any number of args.
Added evaluation of lambdas with any number of args.
Added evaluation of lambdas with variable number of args.
Added evaluation of procedures with var args.
Added evaluation of procedures with any number of args.
Added support for lambdas with no args.
Refactored `define` evaluation to reuse logic from `lambda`.
Refactored application of all kinds of procedures.
Updated TODO.md.
Merge pull request #24 from razielgn/lambdas-with-variable-args

Lambdas with variable args