make value API more generic, allow arbitrary payloads

[tpapp]

Occasionally it is useful to return an extra "payload" along with values and gradients. Examples include

1. generated quantities (eg in Stan),
2. simulated interim data (eg for indirect inference)

Before fixing the API for a longer time, some thought should be given to supporting this.

Here is what I am thinking of:

1. Define accessors get_logdensity, get_gradient, get_payload for values returned by logdensity and logdensity_and_gradient,

2. make the latter two return some lightweight container, which has default methods for the above,

3. with get_payload defaulting to nothing.

The accessors can be introduced at little cost, some dubious workarounds can make the rest non-breaking (dispatch on NTuple) but perhaps a deprecation cycle would be better.

[tpapp]

I experimented with this, and found it impractical. You can always refactor to generate interim values easily from posteriors etc. For small problems the runtime cost is trivial, but for large problems saving interim values that are not needed (eg Hamiltonian steps) would generate a ton of unnecessary allocations.