Merge: json: move up general services, support Byte and more

This PR is a general clean up of the `json` package to prepare the support of two new serialization engines, add small new features and fix some issues in code. It should not change its behavior for existing clients.

* Move up some general deserialization services from `json` to `serialization` in order to make them available to other engines. This includes the class name manipulation services and safety services controlling which types are deserialized, including `class_inheritance_metamodel`.

* Support writing and reading `Byte` as either a plain JSON string or a special object with metadata.

* Support reading `Char` and `Byte` from plain JSON input using simple heuristics.

* Test the deserialization of plain generated JSON and fix a typo in the tests.

* Fix other minor issues in code.

Pull-Request: #2538

diff --cc lib/json/serialization_read.nit
index 03b1267,3f9500a..8dd73f7
--- 1/lib/json/serialization_read.nit
--- 2/lib/json/serialization_read.nit
+++ b/lib/json/serialization_read.nit
@@@ -15,9 -15,12 +15,11 @@@
  # Services to read JSON: `from_json_string` and `JsonDeserializer`
  module serialization_read
  
- import ::serialization::caching
- private import ::serialization::engine_tools
+ import serialization::caching
+ private import serialization::engine_tools
+ import serialization::safe
+ 
  private import static
 -private import string_parser
  import poset
  
  # Deserializer from a Json string.