add ideas list.

ideas.md

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+some ideas for the future:
+
+- things specific to this repl:
+	- saving & loading repl sessions somehow
+	- collections of broadly useful functions that can be loaded into the repl
+- optimization:
+	- lazy evaluation
+	- recognizing when reducing a term forms an endless loop and stopping reduction of that term
+	- trampolining for reduction to prevent stack overflow
+	- memoization
+	- some more efficient way of storing & reducing functions based on finding expressions that occur in several parts of a program? probably related to the above