save cmdarg_stack and isolate command argument state from outer scope
to allow method definition in cmdarg

import from ruby/ruby@04bb9d6b75a55d4000700769eead5a5cb942c25b

Use boolean macro instead of integer literal in `is_debug_info_defined`.

Allow parenthesed do-block in cmdarg

Flush cmdarg flags inside left-paren in a command argument, to allow
parenthesed `do-block` as an argument without arguments parentheses.
`CMDARG_PUSH(0)` for tLPAREN_ARG is before `CMDARG_PUSH(1)` in
`command_args` due to look-ahead.

Sort out the variables which belongs to 'mrb_callinfo'

Add tests for String#rindex of mruby-string-utf8.

Use RARRAY_* macro instead of accessing RArray field.

Add block argument with Array#uniq! and Array#uniq

Implement String#ord, String#split for mruby-string-utf8

Implement String#index, fixes #2073

Add mrb_regexp_p.

based on a patch from @ksss; close #2071

based on a patch from @ksss; close #2070

various functions should have internal linkage

convert exception message to string

Move src/re.h to include/mruby/re.h . 

fixes: mruby -e "p ArgumentError.new(1)"