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.cpp

@@ -181,10 +181,10 @@ void check_named_katom_span(const Katom& begin, const Katom& end)
 
 spans_t find_spans(
     katom_iter begin, katom_iter 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)
 {
     (void)K::log(4, name);
     bool _dbg = false;
@@ -245,10 +245,10 @@ spans_t find_spans(
 
 spans_t find_spans(
     katom_list& 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,
-    std::string name)
+    const std::string& name)
 {
     return find_spans(katoms.begin(), katoms.end(), level_inc, level_dec, error_exit, name);
 }