Panic if an unsupported type is passed to NewValue. (#35)

Let's fail loudly if an unsupported type is passed to NewValue, so users
don't get confusing errors that are hard to trace to their source.

We can always make this more permissive later.

Author: Paddy

tfprotov5/tftypes/value.go

@@ -64,15 +64,31 @@ type Value struct {
 //
 // * bool for Bool
 //
-// * map[string]Value or map[string]interface{} for Map and Object
+// * map[string]Value for Map and Object
 //
-// * []Value or []interface{} for Tuple, List, and Set
+// * []Value for Tuple, List, and Set
 func NewValue(t Type, val interface{}) Value {
-	// TODO: handle ValueCreator implementations.
-	return Value{
-		typ:   t,
-		value: val,
+	if val == nil || val == UnknownValue {
+		return Value{
+			typ:   t,
+			value: val,
+		}
+	}
+	if creator, ok := val.(ValueCreator); ok {
+		var err error
+		val, err = creator.ToTerraform5Value()
+		if err != nil {
+			panic("error creating tftypes.Value: " + err.Error())
+		}
+	}
+	switch val.(type) {
+	case string, *big.Float, bool, map[string]Value, []Value:
+		return Value{
+			typ:   t,
+			value: val,
+		}
 	}
+	panic(fmt.Sprintf("unknown type %T passed to NewValue", val))
 }
 
 // As converts a Value into a Go value. `dst` must be set to a pointer to a