TWI.C is blocking other interrupts while waiting for stop conditions

Hi,
this is not a real error, but I believe it is an improvement.
I would like to propose the following change: insert here an sei() would enable other interrupts while waiting for the stop condtion. Typicly it wouldn't hurt just to wait a few us but having an I2C chip which uses clock stretching like the BNO055 could cause an interupt blocking for more than 100us. Please consider my proposal. The risk to do the sei is when you use the twi inside an interrupt. so it could come to an dead lock because the new write would wait for the TWI_READY.
```
void twi_stop(void)
{
// send stop condition
TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTO);

// change!!
sei();
//
// wait for stop condition to be exectued on bus
// TWINT is not set after a stop condition!
while(TWCR & _BV(TWSTO)){
continue;
}

// update twi state
twi_state = TWI_READY;
}
```
If you are interested, I made a lot of changes to the I2C section. My solution will only work as a master (but could also expanded to a slave), couldn't be used in an interrupt, but the good things are it doesn't need any aditional buffer, has an simpler interface:
```
void begin(void);
void setClock(uint32_t speed);
void setWaitMode(uint8_t mode); // wait after write or just return and let the interupt state machine do the job
bool getSendActive(void); // check if the write job is done
uint8_t getError(void);
uint8_t sendTo(uint8_t chipAdr, uint8_t regAdr, uint8_t *txBuffer, uint8_t length=1, bool stop=true); // max length =254!!!
uint8_t requestFrom(uint8_t chipAdr, uint8_t regAdr, uint8_t *rxBuffer, uint8_t length=1, bool adrStop=true, bool stop=true);
```
and is in some parts faster.
regards
Andre

Read the twi_stop implementation shown in the issue and trace TWI_READY and interrupt-state handling. Evaluate the proposed sei() during the TWSTO wait, including the stated deadlock risk when TWI is used inside an interrupt. Done means the behavior is resolved and the relevant TWI waiting path is validated.

Written by the indexing model from the issue text.