Add mrb_utf8_from_locale, mrb_utf8_free, mrb_locale_from_utf8, mrb_locale_free. Just works for windows.

 * `mrb_show_version()`
 * `mrb_show_copyright()`

mrb_parser_new() and mrb_generate_code() may return NULL.

`PATH_MAX` doesn't have to be defined and if it is, then its value is
usually incorrect (possible buffer overflow).
Use `malloc` and `snprintf` instead to prevent this.

https://www.gnu.org/software/hurd/community/gsoc/project_ideas/maxpath.html

Fix compilation fail caused by GNU Readline and `ISPRINT`.
Also add missing `unsigned char` cast.

also allow spaces around quit/exit.

It doesn't matter to me if one is using FALSE/TRUE instead of 1/0
but I prefer a type (alias) which emphasizes boolean vars to int.
I changed 1/0 to FALSE/TRUE anyway.

remove unused and unneeded:
- sysexit_status
- type (a global variable)

add mrb_ prefix to:
- codedump_all
- class_instance_method_list
- parser_dump

make various functions static, incl.:
- yyparse
- make_exception

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)

currently there are two scnenario to call mrb_run(), the first is
calling a proc, in this case mrb should create a new environment,
discarding all the variables except args, reciever and block.

the second is calling the newly generated irep, like in mirb.
in this case, the variables should be kept after mrb_run().
so we introduce mrb_context_run() to handle this seperately.

Before:

    > "hi"
    hi
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 1: unterminated string meets end of file

After

    > "hi"
    hi
    > d
    (mirb):2: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):3: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 5: unterminated string meets end of file