Use memchr for performance

[ksss]

Use memchr for performance.

This code was copy from https://github.com/ruby/ruby/blob/2f2a5c3ae9f9e7a4d09b1500de8d864f48d69cad/re.c#L264-L269

benchmark

$ uname -v
Darwin Kernel Version 15.2.0: Fri Nov 13 19:56:56 PST 2015; root:xnu-3248.20.55~2/RELEASE_X86_64

s = "b"
str = ("a" * 100 + s)

t = Time.now
str.index(s)
puts Time.now - t

before => 0.000788
after => 0.000508

---

s = "b"
str = ("a" * 100 * 1024 * 1024 + s)

t = Time.now
str.index(s)
puts Time.now - t

before => 0.225474
after => 0.008658

[mattn]

@matz this should work well because this is binary comparing. do you expect mruby-string-ext should work as multi-byte? for example, "あ".index("\x81") should be 0 if string is utf-8.

[mattn]

@matz this should work well because this is binary comparing. do you expect mruby-string-ext should work as multi-byte? for example, "あ".index("\x81") should be 0 if string is utf-8.