Provide the ToExpressionString construction expression for debugging and testing purposes

[dadhi]

I have implemented it via CodeString for LightExpression but nothing prevents its implemention for the System.Linq.Expressions.
The plan is to move the code to the static classavailable both in FEC and FEC.LE.
Pity that we do not have extensions properties, so until C# 9 you will need an extra step to call the method or to use a LightExpression. It is faster anyway.

[dadhi]

Anyway, it seems we need to pass the collected expressions into ToCodeString(Map<Expression, string> processed) as well as to ToExpression to get a fair conversion.

[dadhi]

We may implement the property CodeString as:

string CodeString => ToCodeString(precessed: null)

So the CodeString makes sense for the whole expression.

[dadhi]

Using GetDeclaredMethod(name) and alike to get less fragile name in the generated output, instead of just an array index.

[dadhi]

completed