Refactor fold and walk APIs.

This change moves fold APIs into the `walk` module, integrates folding
with `BranchSequencer` (previously `Enqueue`), and implements the
`starting` and `ending` iterators via `BranchSequencer` instead of
`filter_tree_map`.

With these changes, `has_root` has also been reimplemented and now
returns `When` instead of `bool`.

src/lib.rs

@@ -76,7 +76,7 @@ use thiserror::Error;
 use crate::encode::CompileError;
 use crate::rule::{Checked, RuleError};
 use crate::token::{
-    ConcatenationTree, ExpressionMetadata, ParseError, Text, Token, TokenTree, Tokenized,
+    ConcatenationTree, ExpressionMetadata, ParseError, Text, Token, TokenTree, Tokenized, When,
 };
 #[cfg(feature = "walk")]
 use crate::walk::WalkError;
@@ -750,12 +750,10 @@ impl<'t> Glob<'t> {
     /// As with Unix paths, a glob expression has a root if it begins with a separator `/`.
     /// Patterns other than separators may also root an expression, such as `/**` or `</root:1,>`.
     pub fn has_root(&self) -> bool {
-        self.tree
-            .as_ref()
-            .as_token()
-            .concatenation()
-            .first()
-            .map_or(false, Token::has_root)
+        match self.tree.as_ref().as_token().has_root() {
+            When::Always => true,
+            _ => false,
+        }
     }
 
     /// Returns `true` if the glob has literals that have non-nominal semantics on the target

src/rule.rs

@@ -7,6 +7,7 @@
 //!
 //! Most rules concern alternatives, which have complex interactions with neighboring tokens.
 
+// TODO: Use the term "branch" instead of "group".
 // TODO: The `check` function fails fast and either report no errors or exactly one error. To
 //       better support diagnostics, `check` should probably perform an exhaustive analysis and
 //       report zero or more errors.