Replace educe with a custom derive macro

[sfackler]

Before this PR

Conjure-defined types using f64 need to implement traits that they would not normally be able to using Rust's built-in derive. We previously used the educe crate to be able to customize the behavior of fields containing f64. However, educe is unmaintained and has some significant correctness issues.

After this PR

==COMMIT_MSG==
Replaced educe with a custom derive macro.
==COMMIT_MSG==

To minimize the complexity of the macro, we proxy the trait impls through a delegate type. This allows us to rely on Rust's built-in derive logic for all of the heavy codegen lifting:

#[derive(DeriveWith)]
#[derive_with(PartialEq)]
struct MyType(#[derive_with(with = conjure_object::private::DoubleWrapper)] f64);

// expands to (more or less)

const _: () = {
    #[derive(PartialEq)]
    struct __MyTypeDelegate<'a>(DoubleWrapper<&'a f64>);

    impl<'a> From<&'a MyType> for __MyTypeDelegate<'a> {
        fn from(v: &'a MyType>) -> Self {
            __MyTypeDelegate(DoubleWrapper(&v.0))
        }
    }

    impl PartialEq for MyType {
        fn eq(&self, other: &Self) -> bool {
            __MyTypeDelegate::from(self) == __MyTypeDelegate(other)
        }
    }
};

Possible downsides?

The conversions through the delegate type may make these trait implementations slower.

Since the derive is only intended for use by conjure_codegen, the implementation cuts some corners. For example, it doesn't work on any generic types.

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Description

Replaced educe with a custom derive macro.

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🐛 Fixes

• Replaced educe with a custom derive macro. (#508)