Add do_call macro, implement Bool builtins, add tests

* I noticed that `fn call(...)` in all objects was identical, so I made
  a macro for it. This should make things a little easier to read, since
  do_call is about 30 lines a pop.
* Bool has a constructor now, and a to_int and to_float implementations
* Add tests for constructors and add new bool tests

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

tests/str.npp

@@ -33,8 +33,17 @@ println(a + a)
 println(a + a + a)
 println(a + b)
 println(b + a)
+
 # __mul__
 println("__mul__")
 println(a * 4)
 println(b * 5)
 println((a * 6).to_repr())
+
+# constructor
+println("constructor")
+println(Str("asdf"))
+println(Str(1234))
+println(Str(1.0))
+println(Str(true))
+println(Str(false))