Make default packing configurable

[lexmag]

The idea is to support :packer option in pack/2 that defaults to Msgpax.Packer.
This way any default packing can be overwritten with:

defmodule MyPacker do
  use Msgpax.Packer

  def pack(value) when is_float(value) do
    # pack into 32-bit format
  end
end

Where use Msgpax.Packer expands to @behaviour Msgpax.Packer and also defines the @before_compile callback that should inject Msgpax.Packer.pack for catch-all clause.

We still need to think how to efficiently pass the user-provided packer into Msgpax.Packer for usage when unpacking collections.

[elcritch]

It seems like a good way to handle it, but I'm not sure I follow all the details. Would this impose a performance hit on every user of the library? My first thought was to always check an option and do a callback, but that's probably too slow. I haven't used @before_compile before. It would be able to (at compile time) add the user defined protocol handler and override the default one?