`get_size` should return an option instead of panicking.

Currently, `get_size` panics if it is used on an unsupported type. This is not documented anywhere, and confusing. Returning `None`(or better yet, an `Err`) would clearly indicate this is an unsupported operation, allowing for it's more graceful handling. 

Additionally, `get_size` panic's for some commonly used types, including `bool`s . This makes using it cumbersome. 