Add Nil constructor and tests

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

tests/bool.npp

@@ -70,3 +70,4 @@ println(Bool("true"))
 println(Bool(1))
 println(Bool(1.0))
 println(Bool(true))
+println(Bool(nil))

tests/bool.npp.expect

@@ -53,3 +53,4 @@ true
 true
 true
 true
+false

tests/nil.npp

@@ -0,0 +1,16 @@
+# Nil type operator and function tests
+
+a = nil
+
+println("__not__")
+println(!nil)
+println(!!nil)
+
+println("__eq__")
+println(nil == nil)
+
+println("__ne__")
+println(nil != nil)
+
+println("constructor")
+println(Nil())

tests/nil.npp.expect

@@ -0,0 +1,9 @@
+__not__
+true
+false
+__eq__
+true
+__ne__
+false
+constructor
+nil