Add some enum generic constraints

[roji]

These are contexts which would fail at runtime if a non-enum type is used; the constraint moves the error to compile-time.

[ajcvickers]

Like the idea, but looks like there are build failures. Also, target rc2? (release-8.0)

[roji]

@ajcvickers done, we still need the struct constraint in addition to Enum to exclude the System.Enum class.