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)

sks/kutil/klammer_base.cpp

@@ -5,7 +5,7 @@
 #include "klammer_base.h"
 #include "show.h"
 
-std::string Klammer_base::get(std::string name)
+std::string Klammer_base::get(const std::string& name)
 {
     //std::cout << "GET " << name << "\n";
     std::string result = m_machine.m_state.value(name);
@@ -49,7 +49,7 @@ std::string Klammer_base::result()
         throw Target_error("Unknown target: " + target);
 }
 
-void Klammer_base::show(std::string klammer_name)
+void Klammer_base::show(const std::string& klammer_name)
 {
     std::cout << "Arguments of klammer \"" << klammer_name << "\"\n"
               << m_machine.m_state.describe() << "\n";