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.cpp

@@ -7,14 +7,14 @@
 #include "file.h"
 #include "show.h"
 
-Eval_cpp::Eval_cpp(Machine& machine, Locator loc)
+Eval_cpp::Eval_cpp(Machine& machine, const Locator& loc)
     : m_machine(machine)
     , m_loc(loc)
 {
     (void)K::log(3);
 }
 
-std::string Eval_cpp::eval(fs::path library_path, std::string function_name)
+std::string Eval_cpp::eval(const fs::path& library_path, const std::string& function_name)
 {
     (void)K::log(3, library_path, function_name);
     // msg() << "library path: " << library_path.string().c_str() << "\n";