Get rid of pb_bytes_ptr_t, just allocate pb_bytes_array_t dynamically.

Get rid of pb_bytes_ptr_t, just allocate pb_bytes_array_t dynamically.

This makes the internal logic much simpler, and also keeps the datatypes
more similar between STATIC/POINTER cases. It will still be a bit cumbersome
to use because of variable length array member. Macros PB_BYTES_ARRAY_T(n) and
PB_BYTES_ARRAY_T_ALLOCSIZE(n) have been added to make life a bit easier.

This has the drawback that it is no longer as easy to use externally allocated
byte array as input for bytes field in pointer mode. However, this is still
easy to do using callbacks, so it shouldn't be a large issue.