Enabled composed/nested structs by impl'ing Deserialize ourselves

[FuegoFro]

What did you implement:

This enables Recap to work for multiple layers of nested structs, as seen in the custom_deserialize_allows_nested_structs test. The impetus for this came from things like https://adventofcode.com/2023/day/19, where parsing the workflow with rules with optional conditions gets much easier if you can just describe it as 3 nested structs.

There's details on the implementation written in the derive_impl_deserialize function, but the high level is that, when enabled, this will create a helper struct that mirrors the original one and then handle most deserialization requests by first deserializing into the helper struct and then moving the data to an instance of the original struct. The main difference is custom behavior for str, which allows us to parse the str using the regex and then forward to the helper struct.

Note that currently the Deserialize implementation has some limitations like not working with zero-copy structs (structs that have lifetimes) or generic types (though generics already seems to not work with the existing Recap derivation). This also changes the Recap proc-macro to allow/handle serde attributes, which could lead to false negatives where people are putting attributes that end up being unused and not getting compiler warnings for it when the handle_deserialize attribute is not present. The only way around this that I'm aware of would be to use a different derive name (eg something like #[derive(RecapDeserialize)]) which I'm certainly open to as an alternative to the current #[recap(handle_deserialize)] approach if you'd prefer.

Closes: https://github.com/softprops/recap/issues/10

How did you verify your change:

This also adds integration tests for the derive definition and updates the documentation to describe the new behavior.

What (if anything) would need to be called out in the CHANGELOG for the next release:

The new behavior accessible via the #[recap(handle_deserialize)] attribute and likely the change in allowing serde attributes as well.