compiler: add tests for function literal

Add test forgotten in #934.

pkg/compiler/lambda_test.go

@@ -0,0 +1,15 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestFuncLiteral(t *testing.T) {
+	src := `package foo
+	func Main() int {
+		inc := func(x int) int { return x + 1 }
+		return inc(1) + inc(2)
+	}`
+	eval(t, src, big.NewInt(5))
+}