printf: cast variables to the expected type

use MRB_STRINGIZE

It's defined in mruby/version.h which gets included in mruby.h, so it's
safe to use.

%x expects unsigned int and %p expects void *

GCC emits a diagnostic about %p/void* in pedantic mode:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26542

Remove spaces in end-of-line.

Fix and add test for Numeric#pow behavior.

 * Before

2 ** -1 #=> 0

 * After

2 ** -1 #=> 0.5

Use sizeof() instead of strlen().

Fix build failed on VS2012

Remove a discarded comparison.

Unify include guard styles and header comments.

use mrb_str_cat_lit() instead of mrb_str_cat_cstr().

changed to call check_cv_name_str in check_cv_name_sym and adjust indent

mrb_method_search() does not return NULL.
Instead it raises an exception.
So it is not necessary to evaluate
a return value of mrb_method_search().

Add API `mrb_data_init` to initialize `MRB_TT_DATA` tagged instance.

Remove empty lines.

Use specified macro(NULL) instead of magic-number.

Fix error handling for mrb_pool_realloc().

Fix error handling for mrb_open_allocf().

Fix error handlings for mrb_open_core().

When DISABLE_GEMS is not defined and
a return value of mrb_open_core() is NULL,
mrb_open_allocf() may cause SEGV.

mrb_pool_alloc() may return NULL.

Fix error hanldlings in read_irep_record().

Fix error handling for mrb_generate_code().

The return value of mrb_generate_code()
must be null-checked before referencing.