fix splat without assignment

The parser generates NODE_NIL for tSTAR without argument in masgns. The
codegen didn't handle that.

define YYDEBUG macro conditionally

Defining static version of frexpl also fro MIPS platforms.

On MIPS/linaro, libm.so lacks frexpl() as CygWin does.

The YYDEBUG macro enables parser debugging which
unnecessarily increases the executable size (9 to 10 KB).
Now it only will be defined when PARSER_DEBUG is too.

src/load.c: fix size error macros

parser: fix possible null dereferences

prevent accidental macro redefinition of strndup()

mrb_read_irep_file(): unify error handling / fix uint underflow

Previously there wasn't a way to trigger the useful definition, but it
is needed and that in every case to check for integer overflow due to
multiplication and faulty/modified binary files.

Also change existing code to a calloc()-like parameter order.

It's only used once and in that case it isn't necessary anyway, since
size_t must be at least 32 bits wide and
the max. length of a filename is UINT16_MAX.

parser_state->locals might be a null pointer.

Fixes (and other similiar invalid code):
def a; Proc.new do def x==x end end

buf_size has to be greater than header_size, otherwise subtracting
header_size from buf_size will cause an integer underflow.

Being equal to header_size is fine, however useless, so quit early.

Convert mrb_read_irep_file() to use goto like read_section_lv() and
read_section_debug() already do.

Fix misspellings in document

Add rake task for benchmarking

Use builtins for overflow math if possible

Suppress warnings generated by -Wwrite-strings

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