mpp: introduce object encoding/decoding

[alyapunov]

Now it became possible to pass an object of any class to
mpp::encode and mpp::decode if a co/dec rule is supplied.

There are two ways to specify a rule for a particular class:

• through static member of the class, that is mpp member for
  ecoding/decoding, mpp_enc for encoding only and mpp_dec fo
  decoding only.
• via partial specialization of special global class, that is
  mpp_rule for ecoding/decoding, mpp_enc_rule for ecoding and
  mpp_dec_rule for decoding. In any of these cases a static
  member value is expected in the specialization.

In any case the value of the encoding rule must be a constexpr
expression that could contain any values, including standard
aggregates like std::tuple and pointers to members of the class
that must be encoded/decoded.

Note that one can specify one rule for encoding/decoding as well
as different rules for encoding and decoding.

Examples:

struct IntegerWrapper {
	int i;
	static constexpr auto mpp = &IntegerWrapper::i;
};

An object of such a class will be encoded/decoded as one integer
(MP_INT) value.

struct Triplet {
	int a;
	double b;
	std::string c;
};

template <>
struct mpp_rule<Triplet> {
	static constexpr auto value = std::make_tuple(&Triplet::a,
						      &Triplet::b,
						      &Triplet::c);
};

An object of this class will be encoded/decoded as array (MP_ARR)
of three values - a, b, c - with the corresponding types.

struct Error {
	int code = 0;
	std::string descr;

	static constexpr auto mpp = std::make_tuple(
		std::make_pair(0, &Error::code),
		std::make_pair(1, &Error::descr));
};

This error will be encoded/decoded as map (MP_MAP) with integer
keys (0, 1) and corresponding values (code, descr).