bug: Generic structs error in the idl if they're populated with custom structs

[cryptopapi997]

The title says it all - if we do:

#[account]
struct MyAccount {
    data: GStruct<u32>,
}

#[derive(AnchorSerialize, AnchorDeserialize, Clone)]
struct GStruct<T> {
    data: T,
}

the generated idl is fine and can be used. If we do

#[account]
struct MyAccount {
    data: GStruct<MyStruct>,
}

#[derive(AnchorSerialize, AnchorDeserialize, Clone)]
struct GStruct<T> {
    data: T,
}

#[derive(AnchorSerialize, AnchorDeserialize, Clone)]
struct MyStruct {
    data: u32,
}

the generated idl is unparsable. Might open a PR for this later, but in case I forget wanted to at least have the issue. MRE for this here: https://github.com/cryptopapi997/anchor-generics-mre

[cryptopapi997]

Should be fixed by #3016, feel free to take a look @acheroncrypto