buffer: add Buffer.prototype.lastIndexOf()

* Remove unnecessary templating from SearchString

  SearchString used to have separate PatternChar and SubjectChar template type
  arguments, apparently to support things like searching for an 8-bit string
  inside a 16-bit string or vice versa. However, SearchString is only used from
  node_buffer.cc, where PatternChar and SubjectChar are always the same. Since
  this is extra complexity that's unused and untested (simplifying to a single
  Char template argument still compiles and didn't break any unit tests), I
  removed it.

* Use Boyer-Hoore[-Horspool] for both indexOf and lastIndexOf

  Add test cases for lastIndexOf. Test the fallback from BMH to
  Boyer-Moore, which looks like it was totally untested before.

* Extra bounds checks in node_buffer.cc

* Extra asserts in string_search.h

* Buffer.lastIndexOf: clean up, enforce consistency w/ String.lastIndexOf

* Polyfill memrchr(3) for non-GNU systems

PR-URL: https://github.com/nodejs/node/pull/4846


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>