rework: AST now uses an arena for allocation

For now it works but i dont really like that i use ParseResult, i mean
is necessary but i think i will try to make it cleaner so that i can
just directly use like parse and pass tath into evaluate, that would
require to move the main evaluate funciton into evaluate_tree or
something  and evaluate takes the arena, uses evaluate_tree and frees
the arena, will try that the next commit but for now this version works
perfectly.

src/parser.c

@@ -1,6 +1,7 @@
 #include "parser.h"
 #include "lexer.h"
 #include "arena.h"
+#include <stdalign.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -61,19 +62,33 @@ bool ASTNodeSlice_is_valid(ASTNodeSlice *slice) {
     return true;
 }
 
-AST parse(ASTNodeArray *arr) {
+ParseResult parse(ASTNodeArray *arr) {
     AST tree;
     ASTNodeSlice context = {
         .arr = arr,
         .pos = 0,
     };
+    Arena arena = arena_init(sizeof(ASTNode) * arr->len).arena;
 
-    tree.head = parse_expr(&context, 0);
-    return tree;
+    tree.head = parse_expr(&context, &arena, 0);
+    return (ParseResult) {.arena = &arena, .tree = tree};
 }
 
-ASTNode *parse_expr(ASTNodeSlice *slice, uint8_t min_bp) {
-    ASTNode *left_side = malloc(sizeof(ASTNode));
+ASTNode *parse_expr(ASTNodeSlice *slice, Arena *arena, uint8_t min_bp) {
+    arena_ensure_capacity(
+        arena,
+        sizeof(ASTNode),
+        alignof(ASTNode)
+    );
+
+    ASTNode *left_side = arena_unwrap_pointer(
+        arena_alloc(
+            arena,
+            sizeof(ASTNode),
+            alignof(ASTNode)
+        )
+    );
+
     *left_side = ASTNodeSlice_next(slice);
 
     while (true) {
@@ -90,7 +105,7 @@ ASTNode *parse_expr(ASTNodeSlice *slice, uint8_t min_bp) {
         }
 
         ASTNodeSlice_next(slice);
-        ASTNode *right_side = parse_expr(slice, rbp);
+        ASTNode *right_side = parse_expr(slice, arena, rbp);
 
         ASTNode *new_node = malloc(sizeof(ASTNode));
         *new_node = operator;