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/error.h

@@ -10,8 +10,8 @@ inline std::string command_pathname { "Pathname of command executed on the comma
 
 class Error : std::exception {
 public:
-    Error(std::string error_type, std::string description,
-          Locator locator = Locator(), bool do_justify = true)
+    Error(const std::string& error_type, const std::string& description,
+          const Locator& locator = Locator(), bool do_justify = true)
         : m_type(error_type)
         , m_desc(description)
         , m_loc(locator)
@@ -48,6 +48,13 @@ public:
         : Error("target", description, locator, do_justify) {};
 };
 
+class Klammerset_error : public Error {
+public:
+    explicit Klammerset_error(
+        const std::string& description, const Locator& locator=Locator(), bool do_justify=true)
+        : Error("klammerset", description, locator, do_justify) {};
+};
+
 class Definition_error : public Error {
 public:
     explicit Definition_error(