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/katom_list.h

@@ -23,17 +23,17 @@ find_katom(const std::vector<Katom>::iterator begin, const std::vector<Katom>::i
 
 std::vector<std::pair<Katom, Katom>>
 find_spans(std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end,
-           std::function<bool(const Katom&)> level_inc,
-           std::function<bool(const Katom&)> level_dec,
+           const std::function<bool(const Katom&)>& level_inc,
+           const std::function<bool(const Katom&)>& level_dec,
            bool error_exit,
-           std::string name);
+           const std::string& name);
 
 std::vector<std::pair<Katom, Katom>>
 find_spans(std::vector<Katom>& katoms,
-           std::function<bool(const Katom&)> level_inc,
-           std::function<bool(const Katom&)> level_dec,
+           const std::function<bool(const Katom&)>& level_inc,
+           const std::function<bool(const Katom&)>& level_dec,
            bool error_exit=true,
-           std::string name="all");
+           const std::string& name="all");
 
 
 void describe_spans(const std::vector<Katom>& katoms);