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/eval_cpp.h

@@ -7,12 +7,12 @@
 class Eval_cpp
 {
 public:
-    explicit Eval_cpp(Machine& machine, Locator loc);
+    explicit Eval_cpp(Machine& machine, const Locator& loc);
     Eval_cpp(const Eval_cpp&) = delete;
     Eval_cpp& operator=(const Eval_cpp&) = delete;
     //~Eval_cpp();
 
-    std::string eval(fs::path library_path, std::string function_name);
+    std::string eval(const fs::path& library_path, const std::string& function_name);
 
     Machine m_machine {};
     Locator m_loc;