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/argtype.cpp

@@ -6,9 +6,10 @@
 #include "error.h"
 #include "util.h"
 
-Argtype::Argtype(std::string name, std::string desc, std::string symbolic_pattern, std::string pattern,
-                 std::string default_value, std::string alone_value,
-                 std::string python_cast, modify_string_f python_format,
+Argtype::Argtype(const std::string& name, const std::string& desc, const std::string& symbolic_pattern,
+                 const std::string& pattern,
+                 const std::string& default_value, const std::string& alone_value,
+                 const std::string& python_cast, const modify_string_f& python_format,
                  const Locator& loc)
     : m_name(name)
     , m_desc(desc)