Fix 9819: Allow access to named tuple's names.

Adds a `fieldNames` alias to `std.typecons.Tuple` to return the user-supplied names.  The actual fields are in an expression tuple, so the user-supplied names are simply aliases, which previous to this PR, were not accessible.

https://issues.dlang.org/show_bug.cgi?id=9819

Fix Issue 9819
Would you please change this to a ddoc-ed `unittest` block? That way the example is tested.