Refactored helpers, classes and namespaces for clarity

[ricardoboss]

Things done in this branch:

• Moved all classes from StepLang.Parsing.Statements to StepLang.Statements
• Moved all classes from StepLang.Parsing.Expressions to StepLang.Expressions and all ExpressionResults to StepLang.Expressions.Results
• Renamed ConstantExpression to LiteralExpression (because that's what it actually is)
• Moved helpers for creating BinaryExpressions to the BinaryExpression class
• Moved helpers for creating UnaryExpressions to the UnaryExpression class
• Added explicit NotEquals helper to BinaryExpression (was a compound expression using Not and Equals which resulted in a UnaryExpression)
• Removed the Expression.Constant helpers and replaced them with LiteralExpression.<type> calls
• Removed return void from FunctionResult.VoidFunction so now it is just an empty function (implicitly returning void)
• Renamed ToExpression helper to ToLiteralExpression in ExpressionResult

All in all these are some changes I had in mind for a long time.