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