Phase modifying the AST to explain assets when they fail

Property definitions

nitc :: explain_assert $ ToolContext :: explain_assert_phase
	# Phase modifying the AST to explain assets when they fail
	var explain_assert_phase: Phase = new ExplainAssertPhase(self, [modelize_class_phase, typing_phase, literal_phase])
src/frontend/explain_assert.nit:43,2--44,116