Added support of unmarshalling and marshalling of pointers

[danielbairstow97]

During development of my own project I found some issues when trying to marshal php Nulls N into any Go struct fields.

Was seeing uncaught panics such as reflect: call of reflect.Value.Set on zero Value if the value trying to be set on a struct field was Null.
Additionally if a value was being set on a struct field of type reflect.Ptr that was nil a panic would occur when calling the .Set method on the struct field e.g. reflect.Set: value of type string is not assignable to type *string

I've added testing to showcase the now supported functionality:

• When serializing if a pointer value is Nil the MarshalNil function will be called
• When unserializing if the value passed in is a nil interface the struct field will be left unset (set as default value) rather then panic
• If unmarshalling to a pointer struct field, the field will first be instantiated before attempting to set the value of the pointer

I used this Go playground to try base behaviour off of the encoding/json package: https://go.dev/play/p/5td0XqinlIP

[elliotchance]

Awesome work, thanks! https://github.com/elliotchance/phpserialize/releases/tag/v1.4.0