feat!: Indexes for nodes in the AST (#414)

Indexes allow associating extra data with nodes in the AST
using an array and then quickly looking up the array to fetch
the extra data.

- Index eidx for expressions
- Index sidx for statements
- Index qidx for queries.

AST nodes are not cloneable. Therefore once a module is created,
it is not possible to accidentally create two nodes with the same
index inadvertently via clone.

Also added IndexChecker in debug builds. When a module is parsed,
it will assert that indexes have been constructed correctly.

AST Cleanup
- Make literal expressions (null, val, number, string etc) also structs
  to match all other expressions
- Merge True and False nodes into a single Bool node.

Also update dependencies.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/utils.rs

@@ -19,13 +19,13 @@ pub fn get_path_string(refr: &Expr, document: Option<&str>) -> Result<String> {
                 expr = Some(refr);
             }
             Some(Expr::RefBrack { refr, index, .. }) => {
-                if let Expr::String(s) = index.as_ref() {
-                    comps.push(s.0.text());
+                if let Expr::String { span: s, .. } = index.as_ref() {
+                    comps.push(s.text());
                 }
                 expr = Some(refr);
             }
-            Some(Expr::Var(v)) => {
-                comps.push(v.0.text());
+            Some(Expr::Var { span: v, .. }) => {
+                comps.push(v.text());
                 expr = None;
             }
             _ => bail!("internal error: not a simple ref {expr:?}"),
@@ -112,7 +112,7 @@ pub fn get_root_var(mut expr: &Expr) -> Result<SourceStr> {
     let empty = expr.span().source_str().clone_empty();
     loop {
         match expr {
-            Expr::Var(v) => return Ok(v.0.source_str()),
+            Expr::Var { span: v, .. } => return Ok(v.source_str()),
             Expr::RefDot { refr, .. } | Expr::RefBrack { refr, .. } => expr = refr,
             _ => return Ok(empty),
         }