Serialising Distributions and Functions

[Hazelfire]

Serialising distributions or functions would be valuable for several purposes:

1. Caching distributions or functions between different executions.
2. Sending distributions or functions to entirely different projects to be loaded and manipulated.

One really good example is my work on GiveWell's CEAs which needs to have distributions / functions pulled from the specific CEAs to be included in the final result.

[OAGr]

We now support some simple serialization of distributions. Serialization of functions is much more gnarly, but I think function imports should be good in most of these cases. We can make this feature more powerful if needed.