Implement new read --null flag

The `--null` flag to `read` makes it split incoming lines on NUL instead
of newlines. This is intended for processing the output of a command
that uses NUL separators (such as `find -print0`).

Fixes #1694.

wcstringutil.h

@@ -0,0 +1,29 @@
+/** \file wcstringutil.h
+
+Helper functions for working with wcstring
+*/
+
+#ifndef FISH_WCSTRINGUTIL_H
+#define FISH_WCSTRINGUTIL_H
+
+#include <utility>
+#include "common.h"
+
+/**
+ typedef that represents a range in a wcstring.
+ The first element is the location, the second is the count.
+*/
+typedef std::pair<wcstring::size_type, wcstring::size_type> wcstring_range;
+
+/**
+   wcstring equivalent of wcstok(). Supports NUL.
+   For convenience and wcstok() compatibility, the first character of each
+   token separator is replaced with NUL.
+   Returns a pair of (pos, count).
+   Returns (npos, npos) when it's done.
+   Returns (pos, npos) when the token is already known to be the final token.
+   Note that the final token may not necessarily return (pos, npos).
+*/
+wcstring_range wcstring_tok(wcstring& str, const wcstring &needle, wcstring_range last = wcstring_range(0,0));
+
+#endif