feat: Added arena_alloc

Added arena_alloc and decided to change the arena pointer type from void *
to uint8_t * because i need to support 1 byte pointer arithmetic, thought of
using uintptr_t but not sure, the support seems really weird from what i read
online so i'm sticking with good old uint8_t * (wich i know works).

src/arena.c

@@ -1,6 +1,7 @@
 #include "arena.h"
 #include <stdbool.h>
 #include <stdlib.h>
+#include <string.h>
 
 ArenaResult arena_init(size_t size) {
 
@@ -43,3 +44,30 @@ void arena_destroy(Arena *arena) {
     arena->size = 0;
     free(arena->data);
 }
+
+ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment) {
+    if (arena == NULL) {
+        ArenaPointer err = {
+            .is_valid = false,
+            .err = ARENA_NULL_ARG,
+        };
+        return err;
+    }
+
+    size_t new_offset = align_arena_offset(arena, alignment);
+    if (new_offset + size >= arena->size) {
+        ArenaPointer err = {
+            .is_valid = false,
+            .err = ARENA_OUT_OF_SPACE,
+        };
+        return err;
+    }
+
+    arena->offset = new_offset;
+
+    ArenaPointer val = {
+        .is_valid = true,
+        .pointer = arena->data + arena->offset,
+    };
+    return val;
+}