Serialization and Deserialization for parser Expression

[JonasHedEng]

I'd like to be able to store a successfully parsed CEL expression by de-/serializing it externally. To be able to use it again, I added the Program::from(Expression).

I tried to check whether there are any issues with serde on the Arc-ed types but didn't find why LALRPOP does the wraps. I assume it's a combination of simplifying lifetimes and that it's needed for (potentially) larger ASTs to avoid copying. In the case of most uses of CEL I would expect serialization to be the overhead either way. Maybe it's possible to put a disclaimer in some good place noting that serializing and deserializing potentially could use up much more memory than before the serialization? Either way, it shouldn't have any semantic impact, right?

[clarkmcc]

Sounds similar to #80 except that this approach would not allow for portability between the Go and Rust interpreters for example. I'm not opposed to this though as a stop-gap measure when cross-interpreter portability is not required.

Can you write some tests doing the serialization both ways?

[JonasHedEng]

Cool! Yeah, I was planning to take a stab at implementing #80 if/when I get some time over. I'll look into adding some tests soon