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/klammer_registry.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "klammer.h"
+#include "target_registry.h"
+
+class Klammer_registry
+{
+public:
+    Klammer_registry() = default;
+    void add(const Argtype_registry& argtypes, const Target_registry& targets,
+             std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end, std::vector<Katom>& katoms);
+    void rationalize(const Target_registry& targets);
+    void check_klammer(const std::string& name, const std::string& target, const Locator& loc) const;
+    const std::vector<Katom>* constant_body(const std::string& name) const;
+    std::string instance_list(int margin) const;
+    std::string describe(int margin=0) const;
+
+    std::map<std::string, Klammer> m_klammers {};
+};