Implement basic realloc and add unit test for it (#27)

## Description

Implement basic realloc and add unit test for it

## Issues

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Closes #24 

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Related: #<issue-number>

## Testing

Realloc unit test

Author: felixnagele

src/realloc.cpp

@@ -5,9 +5,26 @@ namespace mem
 
 void* realloc(void* ptr, size_t new_size)
 {
-    (void)ptr;
-    (void)new_size;
-    return nullptr;
+    if (ptr == nullptr)
+    {
+        return malloc(new_size);
+    }
+
+    if (new_size == 0)
+    {
+        free(ptr);
+        return nullptr;
+    }
+
+    void* new_ptr = malloc(new_size);
+    if (new_ptr == nullptr)
+    {
+        return nullptr;
+    }
+
+    free(ptr);
+
+    return new_ptr;
 }
 
 } // namespace mem

tests/unit/test_realloc.cpp

@@ -0,0 +1,10 @@
+#include "malloc_from_scratch/memory_allocator.h"
+#include "test_utils.h"
+
+int main()
+{
+    void* null_ptr = mem::realloc(nullptr, 64);
+    ASSERT_NOT_NULL(null_ptr);
+
+    TEST_PASS();
+}