prevent accidental macro redefinition of strndup()

mrb_read_irep_file(): unify error handling / fix uint underflow

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

Fix to avoid warning

Add test of negative arguments for Integer#%

fix maximum value of the index when access breakpoints.

remove unnecessary <inttypes.h> inclusion

MSVC 2015 implements inline keyword

The format specifier macros were needed to portably print a mrb_int,
because mrb_raisef() originally called vsnprintf(). It doesn't anymore
since 18b2683b and the mrb_int format
specifier macros are already gone.

Apparently the C compiler of Visual Studio 2015 CTP6 finally implements
inline as inline and not only as _inline and __inline.

mruby-time: call ISO C11's timespec_get() if available

refactor write_pool_block()

No need to write the same assertion in each case (except the default
one). Instead we can assert after the switch statement.