Support decoding user-marshalled objects

[frerich]

This PR implements two enhancements which permit decoding user-marshalled objects (indicated by the U type code):

1. Whenever encountering a user-defined object (e.g. a Ruby date) in a stream, the code now emits a 2-tuple giving the object's Ruby class name (a symbol, e.g. Date) as well as any attributes associated with that class (as written out by the marshal_dump method of the Ruby class). For example, here's the result of parsing the data marshalled by running Marshal.dump(Date.today) in Ruby:

iex(1)> value = <<4, 8, 85, 58, 9, 68, 97, 116, 101, 91, 11, 105, 0, 105, 3, 72, 136, 37, 105, 0, 105, 0, 105, 0, 102, 12, 50, 50, 57, 57, 49, 54, 49>>
<<4, 8, 85, 58, 9, 68, 97, 116, 101, 91, 11, 105, 0, 105, 3, 72, 136, 37, 105,
0, 105, 0, 105, 0, 102, 12, 50, 50, 57, 57, 49, 54, 49>>
iex(2)> ExMarshal.decode(value)
{:Date, [0, 2459720, 0, 0, 0, 2299161.0]}

2. ExMarshal.decode (and ExMarshal.Decoder.decode) now support taking options. The only supported option is called user_object_parsers which is expected to take a map associating Ruby class names (i.e. Elixir atoms) with unary functions processing the attributes written by Ruby. This is useful to customise the handling of user objects such that instead of getting a 2-tuple as shown above, custom Ruby types like Date get automatically converted to appropriate Elixir types. Here's an example showing how it can be used to convert Ruby dates to Elixir dates:

value = ...
iex(3)> ExMarshal.decode(value, user_object_parsers: %{Date: fn [_, julian_day, _, _, _, _] -> Date.from_gregorian_days(julian_day - 1721425) end})
~D[2021-05-20]

Two plausible follow-up improvements come to mind:

• I'm not a ExMemcached user myself, but I suppose it would be plausible to also permit specifying custom parsers via configuration files such that the transcoder of Memcached would transparently map types using user object parsers.
• A new module ExMarshal.CustomParsers or such could be devised which implements custom parser functions for common Ruby types such as Date.

[gaynetdinov]

Hey, sorry for the late reply.
I will merge and release the new version this Friday.
Thanks for your contribution!