Extend function Rewriter.scMatch to handle higher-order patterns.
Extend function Rewriter.scMatch to handle higher-order patterns.

With higher-order pattern matching, we can rewrite with equations like

  map (\x -> f x + g x) xs == map f xs + map g xs

which were not possible before.

Higher-order pattern matching will also be useful for implementing
introduction rules in saw-script.

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