v2.0.0-beta.1

Beta release of version 2.

• glitch filters are now enabled in Slave mode making slave devices more
  resistant to electrical noise
• significantly adjusted signal timings so driver is compliant with
  I2C Specification over the full range of allowed rise times
• fixed bug which caused I2CMaster::finished() to return true before STOP
  was sent to slave.
• you can now enable or disable the internal pullup resistors with
  set_internal_pullups() or setInternalPullups()
• reduced pin drive strength which significantly reduced voltage
  undershoot spikes on falling clock edges
• tested port 2 (pins 24 & 25) on both Teensy 4.1 and Teensy 4.0
• analysed and documented I2C timing
  registers to find out what they really do (as opposed to what the datasheet claims!)
• created a simulator
  to predict the effect of changing I2C configuration registers
• created dedicated circuit board to provide tunable pullups and a reliable setup
• created automated tests to check I2C signal timings
• created automated tests of I2CSlave behaviour