Rename Seq to Group

The proc_macro::TokenTree API used to call these Sequence
but it was renamed to Group because sequences sound flat.
People are used to thinking about parentheses as grouping
things.
diff --git a/src/token.rs b/src/token.rs
index 8c2f2d0..d618da7 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -548,7 +548,7 @@
             _ => panic!("unknown delimiter: {}", delim),
         };
 
-        if let Some(seqinfo) = tokens.seq(delim) {
+        if let Some(seqinfo) = tokens.group(delim) {
             match f(seqinfo.inside) {
                 Ok((remaining, ret)) => {
                     if remaining.eof() {