Added FieldNameTuple to std.traits

Why not just
```D
alias FieldNameTuple = staticMap!(NameOf, FieldTypeTyple!T);
```
Did I miss something... ?

Which begs the general question of the usefulness of this function. I'd expose *only* the trait `NameOf` (just like `ConstOf` and friends). Documentation could shouw an example useage of `names = staticMap!(NameOf, FieldTypeTyple!T)`