add metalang99 testsuite (preprocessor stress testing)

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+# Architecture
+
+_This document describes the high-level architecture of Metalang99._
+
+## Interpreter
+
+The interpreter interprets the core metalanguage described in the [specification].
+
+[specification]: https://github.com/hirrolot/metalang99/blob/master/spec/spec.pdf
+
+### `eval/eval.h`
+
+`eval/eval.h` exposes a single macro `ML99_PRIV_EVAL` which evaluates a given metaprogram. It is implemented as a machine in [continuation-passing style] which is described in the specification too.
+
+[continuation-passing style]: https://en.wikipedia.org/wiki/Continuation-passing_style
+
+### `eval/rec.h`
+
+`eval/rec.h` contains a macro recursion engine upon which everything executes.
+
+## Standard library
+
+The Metalang99 standard library is a set of functions implemented using the core metalanguage. They are located inside corresponding files listed at the [documentation]'s front page.
+
+[documentation]: https://metalang99.readthedocs.io/en/latest/