Add #{items := list()} type for encode/decode

[uwiger]

Add support for decoding sub-structures.

Example:

serialization_template(ping, ?PING_VSN) ->
    [ {versions, [#{items => [ {protocol, binary}
                             , {vsns, [int]}]
                   }]
      }
    , {port, int}
...

Previously, the template supported only [type()], denoting a list where all elements are of the same type. This made it tricky to extend the list pattern. Also, complex subtypes aren't necessarily that descriptive:

capabilities_template(<<"chain_poi">>, ?PING_VSN) ->
    [{chain_poi, #{ items => [ {height, int}
                             , {root_hash, binary}
                             , {genesis, binary}
                             , {top, binary}
                             , {poi, binary} ]}
     }].

The serialization would look like this:

(aeternity@localhost)1> Cs = [{chain_poi,[#{height => 1000, root_hash => <<"rx">>, genesis => <<"gx">>, top => <<"tx">>, poi => <<"px">>}]}].
[{chain_poi,[#{genesis => <<"gx">>,height => 1000,poi => <<"px">>,
               root_hash => <<"rx">>,top => <<"tx">>}]}]
(aeternity@localhost)2> aec_peer_messages:serialize_capabilities(2,Cs).
<<221,220,137,99,104,97,105,110,95,112,111,105,145,208,
  207,130,3,232,130,114,120,130,103,120,130,116,120,130,
  112,...>>
(aeternity@localhost)3> aeser_rlp:decode(v(2)).
[[<<"chain_poi">>,
  <<208,207,130,3,232,130,114,120,130,103,120,130,116,120,
    130,112,120>>]]
(aeternity@localhost)4> [[_,Sub]] = v(3).
[[<<"chain_poi">>,
  <<208,207,130,3,232,130,114,120,130,103,120,130,116,120,
    130,112,120>>]]
(aeternity@localhost)5> aeser_rlp:decode(Sub).
[[<<3,232>>,<<"rx">>,<<"gx">>,<<"tx">>,<<"px">>]]