Refactord Concept class to regroup all builtins properties into a ConceptMetadata class

tests/test_ExactConceptParser.py

@@ -98,7 +98,7 @@ def test_i_can_manage_unknown_concept():
 
     assert not res.status
     assert context.sheerka.isinstance(res.value, BuiltinConcepts.UNKNOWN_CONCEPT)
-    assert res.value.obj == "def concept hello world"
+    assert res.value.body == "def concept hello world"
 
 
 def test_i_can_detect_concepts_too_long():
@@ -108,7 +108,7 @@ def test_i_can_detect_concepts_too_long():
 
     assert not res.status
     assert context.sheerka.isinstance(res.value, BuiltinConcepts.CONCEPT_TOO_LONG)
-    assert res.value.obj == "a very very long concept that cannot be an unique one"
+    assert res.value.body == "a very very long concept that cannot be an unique one"
 
 
 def test_i_can_detect_concept_from_tokens():