msgpack_decimal
https://sr.ht/~nabijaczleweli/msgpack_datetime
https://hexdocs.pm/msgpack_decimal/MsgpackDecimal.html
[Un]pack Elixir :decimal Decimals from/to MessagePack extension fields

msgpack only tries the ext callbacks for tuples
(which is how Erlang structs are implemented) but packs maps (Elixir structs) as maps.
Thus to pack Decimal.new("6.7"), you'd pack it as {MsgpackDecimal, Decimal.new("6.7")},
and you'll get back {MsgpackDecimal, %Decimal{}} in return.
Example, assuming `MsgpackDecimal.extension_type` of 110:
  dt = Decimal.new("206.4")
  :msgpack.pack({MsgpackDecimal, dt}, ext: MsgpackDecimal)
  === <<214, 110, 255, 205, 8, 16>>
  :msgpack.unpack("\\x81\\xC4\\vheading_deg\\xD6n\\xFF\\xCD\\b\\x10", ext: MsgpackDecimal)
  === {:ok, %{"heading_deg" => {MsgpackDecimal, Decimal.new("206.4")}}}

The MsgpackDecimal.StructWrap module contains functions to do this automatically
for terms containing maps and lists:
  :msgpack.pack(de |> MsgpackDecimal.StructWrap.wrap(), ext: MsgpackDecimal)
  === <<214, 110, 255, 205, 8, 16>>
  {:ok, unpacked} = :msgpack.unpack("\\x81\\xC4\\vheading_deg\\xD6n\\xFF\\xCD\\b\\x10", ext: MsgpackDecimal)
  unpacked |> MsgpackDecimal.StructWrap.unwrap()
  === %{"heading_deg" => Decimal.new("206.4")}

The extension type is set from $MsgpackDecimal at build time:
  def deps do
    [
      {:msgpack_decimal, "~> 0.1", system_env: [MsgpackDecimal: "123"]}
    ]
  end
https://hex.pm/packages/msgpack_decimal

A Decimal D can be one of: (https://hexdocs.pm/decimal/Decimal.html)
1. signed NaN
2. signed infinity
3. a number consisting of a sign, coefficient, and exponent such that D = sign * coefficient * 10^exponent

A MsgpackDecimal-encoded Decimal's extension payload can be one of:
1. a one-byte special value
2. the exponent as a raw signed byte, then the signed coefficient as a MessagePacked integer

In case 1, the byte is interpreted as 0bSTTT_TTTT where S is the sign bit and T the type.
Types are 0 for NaN and 1 for infinity. Others are errors.

In case 2, the value of the resulting decimal D is given by D = (signed coefficient) * 10^exponent.

Payloads are distinguished by size: 1 for case 1, ≥2 for case 2. An empty payload is an error.

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