change class argument of mrb_const_defined_at from `struct RClass*` to `mrb_value` to make it consistent with mrb_const_defined; ref #2593

Implement Hash[]

update outdated links to the original program.

if you see compiler errors due to this commit, you'd better to use array-modifying
functions, e.g. mrb_ary_set() or mrb_ary_push(), otherwise you might see nasty
GC bugs in the future.  if you are sure what you are doing, replace `RARRAY_PTR(ary)`
by `mrb_ary_ptr(ary)->ptr`. but be warned.

Add inline to str_with_class().

Fix MSVC C++ ABI build. Close #2588.

Fix default gem loading in `generate_gem_table`.

get rid of shadowing variables (mrbgems)

fix strict aliasing rule violation

Mostly renaming, except that the definition of struct accessor methods
is now done in a new function.