Add 'unreachable' method to express unreachable code explicitly

This is an implementation of crystal-lang#4749.

src/exception.cr

@@ -113,3 +113,11 @@ class DivisionByZero < Exception
     super(message)
   end
 end
+
+# Raised when `unreachable!` is called (It must mean a BUG.)
+#
+# ```
+# unreachable! # raises UnreachableError (BUG: unreachable)
+# ```
+class UnreachableError < Exception
+end

src/kernel.cr

@@ -178,6 +178,14 @@ def abort(message, status = 1) : NoReturn
   exit status
 end
 
+# Expresses an unreachable code explicitly.
+#
+# When it is called, it raises `UnreachableError` with given *message*.
+# However you will never see it if you use this method accurately.
+def unrachable!(message = "unreachable") : NoReturn
+  raise UnreachableError.new("BUG: #{message}")
+end
+
 class Process
   # Hooks are defined here due to load order problems.
   def self.after_fork_child_callbacks