Implement IntoPropValue for types that implement serde::Serialize

[bicarlsen]

I haven't looked deeply into it, but it appears that there is not an implementation of IntoPropValue for types that are serde::Serialize.

I imagine this being not so difficult to implement, and would add quite some value. e.g. It would allow one to pass basic enums as a property by only derive-ing serde::Serialize, as opposed to having to implement IntoPropValue manually, which seems to be the current state of things.