perf(ast): box ImportDeclarationSpecifier enum variants (#3061)

Part of #3047.

As with #3058, it's hard to interpret the benchmark results here. But in
this case I think it's easier to see from "first principles" that this
should be an improvement - `ImportSpecifier` is pretty massive (80
bytes) vs `ImportDefaultSpecifier` (40 bytes), and the latter (e.g.
`import React from 'react'`) is common in JS code.

crates/oxc_ast/src/ast/js.rs

@@ -2373,11 +2373,11 @@ pub struct ImportDeclaration<'a> {
 pub enum ImportDeclarationSpecifier<'a> {
     /// import {imported} from "source"
     /// import {imported as local} from "source"
-    ImportSpecifier(ImportSpecifier<'a>),
+    ImportSpecifier(Box<'a, ImportSpecifier<'a>>),
     /// import local from "source"
-    ImportDefaultSpecifier(ImportDefaultSpecifier<'a>),
+    ImportDefaultSpecifier(Box<'a, ImportDefaultSpecifier<'a>>),
     /// import * as local from "source"
-    ImportNamespaceSpecifier(ImportNamespaceSpecifier<'a>),
+    ImportNamespaceSpecifier(Box<'a, ImportNamespaceSpecifier<'a>>),
 }
 
 // import {imported} from "source"

crates/oxc_parser/src/js/module.rs

@@ -107,10 +107,9 @@ impl<'a> ParserImpl<'a> {
     fn parse_import_default_specifier(&mut self) -> Result<ImportDeclarationSpecifier<'a>> {
         let span = self.start_span();
         let local = self.parse_binding_identifier()?;
-        Ok(ImportDeclarationSpecifier::ImportDefaultSpecifier(ImportDefaultSpecifier {
+        Ok(ImportDeclarationSpecifier::ImportDefaultSpecifier(
-            span: self.end_span(span),
+            self.ast.alloc(ImportDefaultSpecifier { span: self.end_span(span), local }),
-            local,
+        ))
-        }))
     }
 
     // import * as name from "module-name"
@@ -119,10 +118,9 @@ impl<'a> ParserImpl<'a> {
         self.bump_any(); // advance `*`
         self.expect(Kind::As)?;
         let local = self.parse_binding_identifier()?;
-        Ok(ImportDeclarationSpecifier::ImportNamespaceSpecifier(ImportNamespaceSpecifier {
+        Ok(ImportDeclarationSpecifier::ImportNamespaceSpecifier(
-            span: self.end_span(span),
+            self.ast.alloc(ImportNamespaceSpecifier { span: self.end_span(span), local }),
-            local,
+        ))
-        }))
     }
 
     // import { export1 , export2 as alias2 , [...] } from "module-name";
@@ -384,7 +382,7 @@ impl<'a> ParserImpl<'a> {
     // ImportSpecifier :
     //   ImportedBinding
     //   ModuleExportName as ImportedBinding
-    pub(crate) fn parse_import_specifier(&mut self) -> Result<ImportSpecifier<'a>> {
+    pub(crate) fn parse_import_specifier(&mut self) -> Result<Box<'a, ImportSpecifier<'a>>> {
         let specifier_span = self.start_span();
         let peek_kind = self.peek_kind();
         let mut import_kind = ImportOrExportKind::Value;
@@ -415,7 +413,12 @@ impl<'a> ParserImpl<'a> {
             let imported = IdentifierName { span: local.span, name: local.name.clone() };
             (ModuleExportName::Identifier(imported), local)
         };
-        Ok(ImportSpecifier { span: self.end_span(specifier_span), imported, local, import_kind })
+        Ok(self.ast.alloc(ImportSpecifier {
+            span: self.end_span(specifier_span),
+            imported,
+            local,
+            import_kind,
+        }))
     }
 
     // ModuleExportName :

crates/oxc_transformer/src/helpers/module_imports.rs

@@ -85,7 +85,7 @@ impl<'a> ModuleImports<'a> {
         names: std::vec::Vec<NamedImport>,
     ) -> Statement<'a> {
         let specifiers = self.ast.new_vec_from_iter(names.into_iter().map(|name| {
-            ImportDeclarationSpecifier::ImportSpecifier(ImportSpecifier {
+            ImportDeclarationSpecifier::ImportSpecifier(self.ast.alloc(ImportSpecifier {
                 span: SPAN,
                 imported: ModuleExportName::Identifier(IdentifierName::new(
                     SPAN,
@@ -96,7 +96,7 @@ impl<'a> ModuleImports<'a> {
                     self.ast.new_atom(name.local.unwrap_or(name.imported).as_str()),
                 ),
                 import_kind: ImportOrExportKind::Value,
-            })
+            }))
         }));
         let import_stmt = self.ast.import_declaration(
             SPAN,