Removed the type definition for defun in parsing. Type will be resolved while invoking instead.
Support for several expressions in the same program
Writing your own functions works
Defining functions and nesting functions works
Back to only working with Ast and not Ast List
Some cleanup in invoker.fs
Small fixes
Merge branch 'Step4'

Conflicts:
	Source/Lisp/Tests/invoker.fs
	Source/Lisp/Tests/parser.fs
	Source/Lisp/invoker.fs
	Source/Lisp/parser.fs
	Source/Lisp/parser.fsy