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>.

crates/proc_macro_srv/src/tests/utils.rs

@@ -52,7 +52,7 @@ pub fn assert_expand(
     let expander = dylib::Expander::new(&path).unwrap();
     let fixture = parse_string(ra_fixture).unwrap();
 
-    let res = expander.expand(macro_name, &fixture.subtree, None).unwrap();
+    let res = expander.expand(macro_name, &fixture.into_subtree(), None).unwrap();
     assert_eq_text!(&expect.trim(), &format!("{:?}", res));
 }