use the new specifier b instead of o
supress cast warning on C++.
Fix disappearance of command_call in case of statement without parentheses.
mrb_field_write_barrier paints also gray in the major collector.
These objects will be traversed after a minor collect cycle.
check if array shrinks in each block
check if array expands in each block
save/restore arena index in mirb; close #740
Merge pull request #744 from authorNari/fixed-field-write-barrier

mrb_field_write_barrier paints also gray in the major collector.
Merge pull request #739 from cremno/use-b-specifier

Use new mrb_get_args binary specifier.
Merge pull request #742 from mattn/supress_cast_warnings

supress cast warning on C++.
Merge pull request #743 from iij/pr-fix-masgn

Fix disappearance of command_call in case of statement without parentheses