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/font_store.cpp

@@ -36,7 +36,7 @@ static std::string lower_extension(const fs::path& path)
 }
 
 
-std::string name_to_dirname(std::string name)
+std::string name_to_dirname(const std::string& name)
 {
     std::string result {};
     for (char c : name) {
@@ -206,7 +206,7 @@ static void extract_font_metrics(Resolved_font& font)
     }
 }
 
-Resolved_font resolve_font(std::string family_name)
+Resolved_font resolve_font(const std::string& family_name)
 {
     if (family_name.empty())
         return {};