Enhancement - Add support for writing/reading multiple I2S samples

[earlephilhower]

PR #2501 had some good ideas which were integrated via later reworks of the I2S subsystem. The one thing that's still missing is a call to the I2S subsystem which can read or write multiple samples at a time.

This is important because most audio codecs can produce many samples at a time, and we can reduce CPU usage significantly by simply writing as many of those samples at a time as we have I2S buffer space. Instead of calling an add-sample() call 44,100 times per second, this would allow us only to have <1/100th those number of calls.

[earlephilhower]

Closed by #5349