`nullSink` was added to `std.range`

$(REF nullSink, std, range) is a convenience wrapper for $(REF NullSink, std,range)
and creates an output range that discards the data it receives. It's the range analog
of `/dev/null`.

---
import std.csv : csvNextToken;

string line = "a,b,c";

// ignore the first column
line.csvNextToken(nullSink, ',', '"');
line.popFront;

// look at the second column
Appender!string app;
line.csvNextToken(app, ',', '"');
assert(app.data == "b");
---
