Add a test for partial evaluation

This is mostly in case I actually implement better functions of arity
>1, but forget to add this test.

src/tests/partial_eval.exp

@@ -0,0 +1,2 @@
+42
+42

src/tests/partial_eval.lambda

@@ -0,0 +1,5 @@
+let sum = fun x -> fun y -> fun z -> x + y + z in
+let plus2 = sum 1 1 in
+
+let _ = print (sum 30 10 2) in
+print (plus2 40)