Added an AST printer

ASTPrinter.cs

@@ -0,0 +1,46 @@
+using System;
+using System.IO;
+using Finn.AST;
+
+namespace Finn;
+
+class ASTPrinter : IVisitor<string>
+{
+	string print(Expr expr)
+	{
+		return expr.accept(this);
+	}
+
+	private string parenthesize(string name, params Expr[] exprs)
+	{
+		var w = new StringWriter();
+		w.Write($"({name}");
+		foreach (Expr expr in exprs)
+		{
+			w.Write(" ");
+			w.Write(expr.accept(this));
+		}
+		w.Write(")");
+		return w.ToString();
+	}
+
+	public string visitBinaryExpr(Binary expr)
+	{
+		return parenthesize(expr.Op.lexeme, expr.Left, expr.Right);
+	}
+
+	public string visitGroupingExpr(Grouping expr)
+	{
+		return parenthesize("group", expr.Expression);
+	}
+
+	public string visitLiteralExpr(Literal expr)
+	{
+		return expr.Value.ToString() ?? "";
+	}
+
+	public string visitUnaryExpr(Unary expr)
+	{
+		return parenthesize(expr.Op.lexeme, expr.Right);
+	}
+}