I2C GPIO

[chuck-h]

Implement & test I2C write for low-speed GPIO outputs so it's compatible with repetitive I2C reads called from inside stepper interrupt.

Plan: when completing a read operation, check for pending write and initiate it before making I2C status ready for next read op.

[chuck-h]

Plan: Make DevAddrWrite-RegisterAddr-RepeatedStart-DevAddrRead-Read cycle a single transaction in twi.c.

[chuck-h]

Implemented unitary I2C read at 06f9226 , no tests with interleaved I2C output yet

[chuck-h]

Fixed blocking-ness of twi_nonBlockingReadRegisterFrom at fdbf5cc . This had been responsible for some unexpectedly long execution times inside the stepper interrupt.

[chuck-h]

Major I2C rewrite with transaction control blocks. Replaced twi.c with twi_tcb.c, implementing generic round-robin-queued read, write, read-modify-write; also a prioritized queued "quickread" function (good for responding to change-of-state interrupt from MCP23017 I2C GPIO). Commit 9891bf9