Add UART interrupt dispatcher #4745
                
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ESP8266 UART ports share a single interrupt service routine (ISR).
This code adds an interrupt dispatcher that allows to define a
different ISR for each UART port.
Why?
NeoPixelBus can use an ISR to send data though Serial1 asynchronously. But it only works when Serial0 is configured as TX_ONLY. Otherwise NeoPixelBus and Serial-RX ISRs conflicts and the ESP8266 reboots. This commit allows both ISRs to coexist.