fix/refactor: Modified evaluate and changed it to evaluate_tree

So i did what the last commit said, also fixed parse_expr because it was
still using malloc for allocating new nodes so i made it use arena_alloc
like it should, did the very first tests so it's all good, i think is
readdy to merge.

include/parser.h

@@ -15,8 +15,8 @@ typedef struct {
 } ASTNodeSlice;
 
 typedef struct {
-    Arena *arena;
-    AST tree;
+    Arena arena;
+    ASTNode *tree;
 } ParseResult;
 
 ASTNode ASTNodeSlice_peek(ASTNodeSlice *slice);