Fix access levels of extension methods

[zspitz]

• Files shouldn't have the suffix Extension -- e.g. LambdaExpression.cs, not LambdaExpressionExtensions.cs
• Class names should have the Extension suffix -- LambdaExpressionExtensions
• Namespace should be ExpressionTreeToString.Util
• Classes and methods should be public; they all exist within their own namespace, so Intellisense polluting is not an issue
• Move any appropriate extension methods to ZSpitz.Util

[zspitz]

Not going to fix the last two.