Main reason for expanding buffer protocol API was to support writes to a
buffer in ADC module (see read_timed).  With this change you can now
create an array of arbitrary type and ADC.read_timed will store into
that array in the correct format (byte, int, float).  I wonder though if
all these changes were really worth it to support just this function.
Hopefully this enhanced buffer protocol API (with typecode specified)
will be used elsewhere.