- String#ljust and String#rjust are now C functions to improve performance
 - infinite loop because of an empty padding argument is now prevented (ArgumentError is raised)
 - extra tests for ljust/rjust added

The new API is:

int mrb_range_beg_len(mrb, range, &beg, &len, len, trunc)

The new argument `trunc` is a boolean value that specifies
whether the function truncates the range. The new return value
is an integer instead of a boolean, that is:

 0: not a range
 1: range with proper edges
 2: out of range

To get the old behavior, you have to rewrite:

  mrb_range_beg_len(mrb, range, &beg, &len, len)

to:

  mrn_range_beg_len(mrb, range, &beg, &len, len, TRUE) == 1

[Breaking Change]

Fix #3357

Prevents RCE
Reported by https://hackerone.com/raydot

Reported from from Alex Snaps via Mathieu Leduc-Hamel,
both from shopify.com.  Thank you!

This reverts commit 5cdcce8d.

define MRB_UTF8_STRING (in mrbconf.h) to enable UTF-8 support.

Not needed anymore since 85075bef

Here are suppressed warnings:

    src/fmt_fp.c: In function 'fmt_fp':
    src/fmt_fp.c:124:16: warning: initialization discards 'const' qualifier from pointer target type
         char *ss = (t&32)?"inf":"INF";
                    ^
    src/fmt_fp.c:125:17: warning: assignment discards 'const' qualifier from pointer target type
         if (y!=y) ss=(t&32)?"nan":"NAN";
                     ^
    mrbgems/mruby-string-ext/src/string.c: In function 'mrb_str_succ_bang':
    mrbgems/mruby-string-ext/src/string.c:302:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "1";
                               ^
    mrbgems/mruby-string-ext/src/string.c:305:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "a";
                               ^
    mrbgems/mruby-string-ext/src/string.c:308:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "A";
                               ^
    mrbgems/mruby-bin-mruby/tools/mruby/mruby.c: In function 'main':
    mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:213:13: warning: assignment discards 'const' qualifier from pointer target type
         cmdline = args.cmdline ? args.cmdline : "-";
                 ^
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c: In function 'print_breakpoint':
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type
       char* enable_letter[] = {BREAK_INFO_MSG_DISABLE, BREAK_INFO_MSG_ENABLE};
       ^
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type

use flags 4 for *this object is embed*
use flags 8~64 for *embed string length*

- 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)