Klammerset: the @@@klammerset construct, its search path, and const correctness

The @@@klammerset system command formally declares a klammerset — a
named, logically related group of klammer definitions — with an
operative, idempotent declaration (:requires and :files load in order
at the declaration point, relative to the declaring file). A bare
symbol given to ktext -k, kdesc --input, or :requires resolves to
x/x.k on the search path: the document's directory, then
KLAMMERTEXT_KLAMMERSETS, then KLAMMERTEXT_HOME; kdesc --klammerset
lists the available sets. sks/sks.k is the first declared klammerset,
so `-k sks` loads the SKS by name. The engine's lookup classes were
renamed *_set → *_registry to keep the two concepts apart, and the
whole C++ tree now follows standard const-correctness conventions.
tst/ gains klammerset_test.sh (18 cases).

(from dev 64b1abf23e56)

mac/argument.h

@@ -19,11 +19,10 @@ public:
         , m_target()
         {};
 
-    Parameter(const std::string& name, Argtype argtype, const Locator& loc,
-             bool optional=false, std::string default_value = "");
+    Parameter(const std::string& name, const Argtype& argtype, const Locator& loc,
+             bool optional=false, const std::string& default_value = "");
     ~Parameter() = default;
 
-    std::string python_value(std::string value);
     bool undefined() const { return m_name == "_default"; };
 
     std::string m_name {};