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.
Allocation decision is now made before the field data type is decided.
This way the data type decisions can more cleanly account for the allocation
type, i.e. FT_DEFAULT logic etc.
Added pb_bytes_ptr_t for pointer-allocated bytes-fields. There is no point
generating separate structs for these, as they would all be of the same type.
