Added len() function

Closes #120

Now all objects that have __len__() defined can be called with len(), even with classes!
```
class Test
{
    fun __constructor__(){};
    fun __len__()
    {
        return 10
    }
}
len(Test()) # 10
len("test") # 4
len([1,2,3]) # 3
```

core/builtin_funcs.py

@@ -97,6 +97,15 @@ def execute_print(self, exec_ctx: Context) -> RTResult[Value]:
     @args(["value"])
     def execute_print_ret(self, exec_ctx: Context) -> RTResult[Value]:
         return RTResult[Value]().success(String(str(exec_ctx.symbol_table.get("value"))))
+
+    @args(["value"])
+    def execute_len(self,exec_ctx: Context) -> RTResult[Value]:
+        val = exec_ctx.symbol_table.get("value")
+        try:
+            ret = int(len(val))
+            return RTResult[Value]().success(Number(ret))
+        except TypeError:
+            return RTResult[Value]().failure(Error(self.pos_start,self.pos_end,"TypeError","Object has no len()"))
 
     @args(["value"])
     def execute_input(self, exec_ctx: Context) -> RTResult[Value]:
@@ -552,6 +561,7 @@ def create_global_symbol_table() -> SymbolTable:
     ret.set("cls", BuiltInFunction("clear"))
     ret.set("require", BuiltInFunction("require"))
     ret.set("exit", BuiltInFunction("exit"))
+    ret.set("len",BuiltInFunction("len"))
     # Datatype validator methods
     ret.set("is_num", BuiltInFunction("is_num"))
     ret.set("is_int", BuiltInFunction("is_int"))

core/datatypes.py

@@ -133,6 +133,8 @@ class Iterator(Value):
     def __init__(self, generator: Generator[RTResult[Value], None, None]) -> None:
         super().__init__()
         self.it = generator
+
+    def __len__(self): return len(self.it)
 
     def iter(self) -> Iterator:
         return self
@@ -1099,6 +1101,12 @@ class Instance(BaseInstance):
     def __init__(self, parent_class: Class) -> None:
         super().__init__(parent_class, None)
 
+    def __len__(self):
+        try:
+            return self.operator("__len__")[0].value
+        except AttributeError:
+            return Null.null()
+
     def bind_method(self, method: BaseFunction) -> RTResult[BaseFunction]:
         method = method.copy()
         if method.symbol_table is None:

stdlib/array.rn

@@ -22,7 +22,7 @@ class Array {
         return (this.list)[start:end]
     }
 
-    fun len() -> arr_len(this.list)
+    fun __len__() -> arr_len(this.list)
     fun is_empty() -> this.list == []
     fun to_string() -> str(this.list)
     fun is_array() -> true