mbedtls_memory_buffer_alloc_init() function

Initialize use of stack-based memory allocator. The stack-based allocator does memory management inside the presented buffer and does not call calloc() and free(). It sets the global mbedtls_calloc() and mbedtls_free() pointers to its own functions. (Provided mbedtls_calloc() and mbedtls_free() are thread-safe if MBEDTLS_THREADING_C is defined)