Change the syntax for type parameter kinds

[marijnh]

[This was the relevant e-mail I sent to rust-dev. No one objected, several people agreed.]

At least two people have confused @t with a boxed type in the past
weeks. Even me, who really should know better at this point,
occasionally misinterpret some code because of the similarity between
the two. The same probably goes for ~T.

I think we should change the syntax. It can be a little more verbose,
I think, as long as it doesn't resemble other type-related syntax.

I'd also support making 'shared' the default kind for type parameters.
It is usually what you want.

Proposal: 'pinned T' and 'unique T' for pinned/unique type params. 'T'
for shared ones.