Check that #define-d PIN combinations make sense at compile-time

[lurch]

(as initially suggested in #192)

It might be nice to have a compile-time #error if you give an invalid combination of UART_DEFAULT settings? e.g. setting {PICO_DEFAULT_UART=0, PICO_DEFAULT_UART_TX_PIN=13} (invalid because GPIO13 is UART0 RX) or {PICO_DEFAULT_UART=1, PICO_DEFAULT_UART_TX_PIN=0} (invalid because GPIO0 is UART0 TX). (with similar things being done for I2C and SPI pins too)