Rewrite SerialPIO receive path, ensure proper edge

[earlephilhower]

The SerialPIO(SoftwareSerial) receive path was convoluted and required a lot of work on the host to get the actual data out. It also wasn't always sampling on the proper edge leading to errors whenever clocks or hold times shifted slightly.

Rewrite the SerialPIO RX path fo explicitily wait for start bit, pause 1/2 bit time, then idle for a full bit time for all bits. Takes more PIO instruction memory but works flawlessly even with mismatched clocks.

Tested with a loopback from HW UART to SW UART with a 5% clock mismatch @ 19200 baud without reception errors, whereas the original code would fail with less than a 0.5% variation.

Fixes #2928

#include <SoftwareSerial.h>
SoftwareSerial s(15, -1);
void setup() {
  Serial1.setTX(0);
  Serial1.begin(19200 + 1920/2, SERIAL_8N1);
  s.begin(19200, SERIAL_8N1);
}

void loop() {
  Serial.println("---");
  Serial1.write("Had we but world enough and time", 32);
  uint32_t now = millis();
  while (millis() - now < 500) {
    while (s.available()) {
      auto c = s.read();
      Serial.printf("%02x '%c'\n", c, c);
    }
  }
}