allow for `fixed(positive(1.0))` - apply `value` recursively?

[st--]

To allow for specifying both that a parameter should be positive, and is currently not to be optimised over, it'd be great to support fixed(positive(1.0)). This could be achieved by applying value recursively (or at least twice, perhaps special-casing for fixed?).

Happy to provide a PR if you'd be willing to include it.

[willtebbutt]

A PR would be very welcome :)