xo-expression: ++ docs

LESSONS

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+* want reflection to work without requiring std::type_info,
+  so that in xo-expression we can create function types that
+  C++ compiler has never encountered.
+
+** This means we need the ability to construct the canonical name
+   for a type without c++ compiler's assistance.
+
+** need lookup using function ingredients (return types + arg types + noexcept)
+   to intern lambda types

TODO

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+- In xo-reflect, TypeDescr objects are immortal.
+  In original context rationale was recording information for already-established C++ types.
+  Maybe want to revisit that choice when associating TypeDescr objects with expressions?