Fix error handlings in mirb.

mrb_parser_new() and mrb_generate_code() may return NULL.
Fix error hanldlings in read_irep_record().

read_irep_record_1() and read_irep_record() may return NULL.
Fix error handling for mrb_pool_realloc().

mrb_pool_alloc() may return NULL.
Fix error handlings for mrb_open_core().

Conflicts:
	tasks/mrbgems_test.rake
	test/init_mrbtest.c
Use specified macro(NULL) instead of magic-number.

Conflicts:
	src/class.c
Fix Numeric#pow behavior.

 * Before

2 ** -1 #=> 0

 * After

2 ** -1 #=> 0.5

Conflicts:
	src/numeric.c
Add test for Numeric#pow.

Conflicts:
	test/t/numeric.rb
printf: cast variables to the expected type

%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 unused MSVC strtoll fallback
add write barrier to env on pop call info poped. #2525
revert 6c1dfc9; ref #2525 #2565

Conflicts:
	src/proc.c
Fix error handlings for fprintf().

Conflicts:
	src/dump.c
Fix error handlings for fputs().
Fix allocation-error-handlings for scope_new().

Conflicts:
	src/codegen.c
should use non NULL scope for raise_error(); ref #2547