Refactor TokenStream to hold Vec<TokenTree> instead of tt::Subtree

Refactor TokenStream to hold Vec<TokenTree> instead of tt::Subtree

`TokenStream` assumes that its subtree's delimeter is `None`, and this
should be encoded in the type system instead of having a delimiter field
that is mostly ignored.

`tt::Subtree` is just `pub delimiter: Option<Delimiter>, pub
token_trees: Vec<TokenTree>`, so a Subtree that is statically guaranteed
not to have a delimiter is just Vec<TokenTree>.