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)

sks/target/latex_util.h

@@ -6,23 +6,23 @@
 
 namespace latex {
 
-std::string page(std::string structure,
-                 std::string title,
-                 std::string subtitle,
-                 std::string authors,
-                 std::string date,
-                 std::string version,
-                 std::string copyright,
-                 std::string bottom,
-                 std::string prolog,
-                 std::string paper_size,
+std::string page(const std::string& structure,
+                 const std::string& title,
+                 const std::string& subtitle,
+                 const std::string& authors,
+                 const std::string& date,
+                 const std::string& version,
+                 const std::string& copyright,
+                 const std::string& bottom,
+                 const std::string& prolog,
+                 const std::string& paper_size,
                  bool two_column,
                  float leading,
                  int pointsize,
                  bool ragged_right,
-                 std::string cover,
+                 const std::string& cover,
                  bool landscape,
-                 std::string body,
+                 const std::string& body,
                  Resolved_font serif_font = {},
                  Resolved_font sans_font = {},
                  Resolved_font mono_font = {},