Merge: Intro MessagePack serialization engines

Merge: Intro MessagePack serialization engines

MessagePack is a portable data serialization format. It differs from JSON by its binary format, compressing common types, support for custom types and accepting any object as map key. Here, the primitive Nit types use the shortest available MessagePack format (e.g. an `Int` between -32 and 127 takes a single byte). The custom types are used to declare some metadata and special types (objects, references, `Char`, etc.). The engines can, optionally, benefit from the map keys as any object to compress repeated metadata (class and attribute names).

These engines should replace the current `Binary::serialization` services, which are almost impossible to debug, less compressed than MessagePack and not standard.

The serialization engine supports writing and reading plain (no metadata) MessagePack data, similarly to the JSON engines.

Most clients should use the high-level services:
* Customizable engines to handle errors: `MsgPackSerializer` and `MsgPackDeserializer`.
* Quick services on streams: `Writer::serialize_msgpack` and `Reader::deserialize_msgpack`.
* Quick services on `Bytes` (mostly for testing): `Serializable::serialize_msgpack` and `Bytes::deserialize_msgpack`.

Or, to write engines one can use the low-level services:
* Low-level writing methods: `Writer::write_msgpack_uint64`, `Writer::write_msgpack_bin8`, etc.
* Low-level reading method: `Reader::read_msgpack`.

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Using it in Jwrapper, the MessagePack format creates smaller files than JSON (at ~50% the size), when using the default options and serializing the metadata of the Java standard library:

* JSON: 14.9 MB
* MessagePack (default): 7.0 MB
* Old binary: 9.8 MB

The file gets shorter in plain MessagePack mode or when caching the metadata strings:
* MessagePack `plain_msgpack=true`: 6.3 MB
* MessagePack `cache_metadata_strings=true`: 3.0 MB

Caching metadata creates much smaller files, however for the size only gzip has a bigger effect.
Further investigations is required to see the impact it has on the speed.

* gzipped MessagePack (default): 0.7 MB
* gzipped MessagePack `cache_metadata_strings=true`: 0.6 MB

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This PR depends on #2539 as it works on binary data, you can ignore the first 2 commits.

@R4PaSs you may want to look at the commits modifying `Int::to_bytes` and `Bytes::to_i`.

Pull-Request: #2550
Reviewed-by: Jean Privat <jean@pryen.org>