add metalang99 testsuite (preprocessor stress testing)

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+# Optimization tips
+
+_This document describes a few optimization tips when using Metalang99._
+
+Generally speaking, the fewer reduction steps you perform, the faster you become. A reduction step is a concept defined formally by the [specification]. Here is its informal (and imprecise) description:
+
+ - Every `v(...)` is a reduction step.
+ - Every `ML99_call(op, ...)` induces as many reduction steps as required to evaluate `op` and `...`.
+
+To perform fewer reduction steps, you can:
+
+ - use `ML99_callUneval`,
+ - use plain macros (e.g., `ML99_CAT` instead of `ML99_cat`),
+ - use optimized versions (e.g., `ML99_listMapInPlace`),
+ - use tuples/variadics instead of lists,
+ - call a macro as `<X>_IMPL(...)`, if all the arguments are already evaluated.
+
+<details>
+    <summary>Be careful with the last trick!</summary>
+
+I strongly recommend to use the last trick only if `X` is defined locally to a caller so that you can control the correctness of expansion. For example, `X` can become painted blue, it can emit unexpected commas, the `#` and `##` operators can block expansion of parameters, and a plenty of other nasty things.
+</details>
+
+[specification]: spec/spec.pdf