Change the implementation of Arena to use virtual memory, and remove the old gbArena code

src/parser.hpp

@@ -95,7 +95,7 @@ struct AstFile {
 	AstPackage * pkg;
 	Scope *      scope;
 
-	Arena        arena;
+	Arena  arena;
 
 	Ast *        pkg_decl;
 	String       fullpath;
@@ -741,11 +741,10 @@ gb_inline bool is_ast_when_stmt(Ast *node) {
 	return node->kind == Ast_WhenStmt;
 }
 
-gb_global Arena global_ast_arena = {};
+gb_global gb_thread_local Arena global_ast_arena = {};
 
 gbAllocator ast_allocator(AstFile *f) {
 	Arena *arena = f ? &f->arena : &global_ast_arena;
-	// Arena *arena = &global_ast_arena;
 	return arena_allocator(arena);
 }