Draft algorithms for caching a (sub-)graph

[zolkis]

To help clarify the discussion about caching details, scope and reach, draft algorithms that could be used for cache read/write.

During compilation, given a graph:

[algorithm:] check if graph is available/can be used from the cache
- compute hash from graph
- let cached be the result of retrieving hash from the cache
- if that fails, fail
- otherwise return cached.

otherwise,

[algorithm:] if caching is allowed,
[steps]

[algorithm:] insert graph into the cache
- let hash be the result of computing hash from graph
- return hash

(These are just placeholders, feel free to expand / change.)