std.format.formattedRead: fix returned count of parsed variables

Hello,

I'd like to suggest the following patch for "std.format.formattedRead".
It deals with correctly counting the number of parsed variables, when the parameter passed to "formattedRead()" is a tuple.

Current behavior is confusing, and counts all the members of the tuple as 1, regardless of how many were correctly parsed. See the unittests for examples.

Regards,
 -gordon