Add 'unreachable!' method to express unreachable code explicitly

This is an implementation of crystal-lang#4749.

spec/std/unreachable_spec.cr

@@ -0,0 +1,13 @@
+require "spec"
+
+describe "unreachable!" do
+  it "raises UnreachableError with 'BUG: unreachable' message" do
+    expect_raises(UnreachableError, "BUG: unreachable") { unreachable! }
+  end
+
+  it "can set an error message" do
+    expect_raises UnreachableError, "BUG: i'm bag" do
+      unreachable! "i'm bag"
+    end
+  end
+end

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 unreachable!(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