rework mruby-objectspace and gc.[ch]

- functions should have C linkage (for C++ code)
- add prefix to each_object_callback
- use more appropriate variable types / initialization
- ObjectSpace::count_objects has 1 opt. arg.
- prefer mrb_intern_lit to mrb_intern_cstr for str. lit.
- mruby/value.h is included by mruby.h
- adjust coding style

include/mruby/gc.h

@@ -7,11 +7,16 @@
 #ifndef MRUBY_GC_H
 #define MRUBY_GC_H
 
-#include "mruby.h"
-#include "mruby/value.h"
+#if defined(__cplusplus)
+extern "C" {
+#endif
 
-typedef void (each_object_callback)(mrb_state *mrb, struct RBasic* obj, void *data);
-void mrb_objspace_each_objects(mrb_state *mrb, each_object_callback* callback, void *data);
+typedef void (mrb_each_object_callback)(mrb_state *mrb, struct RBasic *obj, void *data);
+void mrb_objspace_each_objects(mrb_state *mrb, mrb_each_object_callback *callback, void *data);
 void mrb_free_context(mrb_state *mrb, struct mrb_context *c);
 
+#if defined(__cplusplus)
+}  /* extern "C" { */
+#endif
+
 #endif  /* MRUBY_GC_H */