simplify stack_clear()
add some methods to Symbol
remove unused value for mrb_gv_remove
add mrb_gv_remove(); untested
Merge pull request #1535 from matsumoto-r/del_unused_varaiable

remove unused value for mrb_gv_remove
Merge pull request #1534 from skandhas/pr-add-some-symbol-methods

Add some methods to Symbol
define Module#const_missing
better error position display
Set regs to stack on return of const_get calls before use
Merge pull request #1538 from carsonmcdonald/conststackfix

Set regs to stack on return of const_get calls before use
move declarations to the beginning of blocks