UART: Always allocate UART objects in the long-lived pool

Particularly when they have buffers that are written via IRQ or DMA,
UART objects do not relocate gracefully.  If such an object is
relocated to the long-lived pool after its original creation, the
IRQ or DMA will write to an unexpected location within the Python
heap, leading to a variety of symptoms.  The most frequent symptom
is inability to read from the UART.

Consider the particular case of atmel-samd: usart_uart_obj_t
contains a usart_async_descriptor contains a _usart_async_device.
In _sercom_init_irq_param the address of this contained
_usart_async_device is assigned to a global array
sercom_to_sercom_dev which is later used from the interrupt context
_sercom_usart_interrupt_handler to store the received data in the
right ring buffer.

When the UART object is relocated to the long-lived heap, there's no
mechanism to re-point these internal pointers, so instead take the
cowardly way and allocate the UART object as long-lived.

Happily, almost all UART objects are likely to be long-lived, so
this is unlikely to have a negative effect on memory usage or heap
fragmentation.

Closes: #1056

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