Remove `tokenstream::Delimited`.

Remove `tokenstream::Delimited`.

Because it's an extra type layer that doesn't really help; in a couple
of places it actively gets in the way, and overall removing it makes the
code nicer. It does, however, move `tokenstream::TokenTree` further away
from the `TokenTree` in `quote.rs`.

More importantly, this change reduces the size of `TokenStream` from 48
bytes to 40 bytes on x86-64, which is enough to slightly reduce
instruction counts on numerous benchmarks, the best by 1.5%.

Note that `open_tt` and `close_tt` have gone from being methods on
`Delimited` to associated methods of `TokenTree`.