Fix where on struct declarations

[Xt-8]

Add dedicated struct declaration that fixes where clause at the end of tuple structs. Also adds type highlighting to where on normal structs to match where on impl.

#131

Edit:

Also adds type highlighting to where on normal structs to match where on impl.

And by this I mean it highlights the types after a struct's where.

[zargony]

Nice work, thanks!

Would this also apply to enum declarations? As far as I know, they can have where statements as well?

[Xt-8]

It was for fixing where on tuple struct specifically. I think the where clause works on enums already, and I don't believe there is a tuple enum. I haven't been using rust for a bit however, so I can't be sure.