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.

read_irep_record_1() and read_irep_record() may return NULL.

Fix error handlings in mirb.

mrb_parser_new() and mrb_generate_code() may return NULL.

refactor mruby method(fix indent. remove temporary value etc)

Improve replacement math functions for Visual C++.

Refactor classc