loci: constify most global variables

The compiler will now put them in rodata, which may be more efficient.
diff --git a/c_gen/c_show_gen.py b/c_gen/c_show_gen.py
index 5dab038..14709ab 100644
--- a/c_gen/c_show_gen.py
+++ b/c_gen/c_show_gen.py
@@ -226,7 +226,7 @@
     # Generate big table indexed by version and object
     for version in of_g.of_version_range:
         out.write("""
-static loci_obj_show_f show_funs_v%(version)s[OF_OBJECT_COUNT] = {
+static const loci_obj_show_f show_funs_v%(version)s[OF_OBJECT_COUNT] = {
 """ % dict(version=version))
         out.write("    unknown_show, /* of_object, not a valid specific type */\n")
         for j, cls in enumerate(of_g.all_class_order):
@@ -243,7 +243,7 @@
         out.write("};\n\n")
 
     out.write("""
-static loci_obj_show_f *show_funs[5] = {
+static const loci_obj_show_f *const show_funs[5] = {
     NULL,
     show_funs_v1,
     show_funs_v2,