Similarly to StrictUnicode mode (see b28613c2) add new opt-in mode that
requests to decode Python dicts as ogórek.Dict instead of builtin map.
As explained in recent patch "Add custom Dict that mirrors Python dict
behaviour" this is needed to fix decoding issues that can be there due
to different behaviour of Python dict and builtin Go map:

    ---- 8< ----
    Ogórek currently represents unpickled dict via map[any]any, which is
    logical, but also exhibits issues because builtin Go map behaviour is
    different from Python's dict behaviour. For example:

    - Python's dict allows tuples to be used in keys, while Go map
      does not (https://github.com/kisielk/og-rek/issues/50),

    - Python's dict allows both long and int to be used interchangeable as
      keys, while Go map does not handle *big.Int as key with the same
      semantic (https://github.com/kisielk/og-rek/issues/55)

    - Python's dict allows to use numbers interchangeable in keys - all int
      and float, but on Go side int(1) and float64(1.0) are considered by
      builtin map as different keys.

    - In Python world bytestring (str from py2) is considered to be related
      to both unicode (str on py3) and bytes, but builtin map considers all
      string, Bytes and ByteString as different keys.

    - etc...

    All in all there are many differences in behaviour in builtin Python
    dict and Go map that result in generally different semantics when
    decoding pickled data. Those differences can be fixed only if we add
    custom dict implementation that mirrors what Python does.

    -> Do that: add custom Dict that implements key -> value mapping with
       mirroring Python behaviour.

    For now we are only adding the Dict class itself and its tests.
    Later we will use this new Dict to handle decoding dictionaries from the pickles.
    ---- 8< ----

In this patch we add new Decoder option to activate PyDict mode
decoding, teach encoder to also support encoding of Dict and adjust
tests.

The behaviour of new system is explained by the following doc.go
excerpt:

    For dicts there are two modes. In the first, default, mode Python dicts are
    decoded into standard Go map. This mode tries to use builtin Go type, but
    cannot mirror py behaviour fully because e.g. int(1), big.Int(1) and
    float64(1.0) are all treated as different keys by Go, while Python treats
    them as being equal. It also does not support decoding dicts with tuple
    used in keys:

         dict    ↔  map[any]any                       PyDict=n mode, default
                 ←  ogórek.Dict

    With PyDict=y mode, however, Python dicts are decoded as ogórek.Dict which
    mirrors behaviour of Python dict with respect to keys equality, and with
    respect to which types are allowed to be used as keys.

         dict    ↔  ogórek.Dict                       PyDict=y mode
                 ←  map[any]any