Can we support Expression on refrence type not only struct

[oshvartz]

We want to validate inputs that are reference type - is there a reason not supporting it
looked at the build in guards and could not find something supporting expression on refence type

[vlmironov]

@ardalis could you elaborate on this one, please? Guard.Against.Expression() seems to support only value types. Any reason for it?
I am currently working around with Guard.Against.InvalidInput() but it doesn't sound too good in my code semantically.