fec: wip: adding async (PDU) encoder and decoder blocks.
digital: adding an async message passsing CRC32 calc/check block for PDUs.
fec: wip: fixing formatting.
digital: added flat to packet_utils.unmake_packet to check or not check the CRC.
blocks: adds kernels for pack_k_bits and unpack_k_bits.
fec: wip: using unpack/pack k bits kernels instead of copying logic in here.

Need to remove volk_malloc from encode.
blocks: adding an option to swap the order of the output bits of a repack_bits block.
digital: don't need the FEC info for the tagged stream corr access code.

Currently, running FEC on payload before header, so the header encodes the fully encoded payload lenth.
fec: wip: adding extended encoder for async version.

not quite working yet, needs work on the message-passing hier_block and added async puncturing block.