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/locator.h

@@ -23,7 +23,7 @@ public:
         , m_line(int(location.line()))
         , m_chr(int(location.column()))
         {};
-    Locator(fs::path filename, int line, int chr);
+    Locator(const fs::path& filename, int line, int chr);
     std::string str(bool relative = false) const;
     std::string desc(bool relative = false) const;
     std::string abbrev(bool include_chr=true) const;