7892: Fix TokenStream::from_str for input consisting of a single group with delimiter r=edwin0cheng a=kevinmehall
TokenStream holds a `tt::Subtree` but assumes its `delimiter` is always `None`. In particular, the iterator implementation iterates over the inner `token_trees` and ignores the `delimiter`.
However, `TokenStream::from_str` violated this assumption when the input consists of a single group by producing a Subtree with an outer delimiter, which was ignored as seen by a procedural macro.
`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>`.
Fixes#7810Fixes#7875
Co-authored-by: Kevin Mehall <km@kevinmehall.net>
f0b7c02f16