For both v1 and v2, provide routines to help do arbitrary length
write/read transfers.
Tested with multiple byte writes and reads, for both 100khz and 400khz,
with repeated starts and stop/starts. However, only tested (presently)
with a single i2c target device, a Sensiron SHT21 sensor. Extended
testing against eeproms and alternative devices would be useful
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