Feature: support explicit parameter bounds with implicit defaults

I have a struct, `Node<T, D>`, where `D` is phantom, and due to some changes to a type `Node` embeds, I now need `Sync + Send + Any` bounds on `T` to get the various `*Eq` and `*Ord` traits on `Node<T,D>`. This resulted in me needing to make the following change: ``` diff -#[derive_where(PartialEq, Eq, PartialOrd, Ord; T)] +#[derive_where(PartialEq; T: PartialEq + Sync + Send + Any)] +#[derive_where(Eq; T: Eq + Sync + Send + Any)] +#[derive_where(PartialOrd; T: PartialOrd + Sync + Send + Any)] +#[derive_where(Ord; T: Ord + Sync + Send + Any)] struct Node<T, D> { ... } ``` Explanation: whereas before I had a single `derive_where` call `#[derive_where(PartialEq, Eq, PartialOrd, Ord; T)]`, now I need a separate `derive_where` call for each of these traits. The reason is that once I add explicit trait bounds `T: Sync + Send + Any`, I also need to specify the default recursive trait bound as well, which of course is different for each trait I'm deriving. **What I'm proposing is some way to say "I'd like the default bounds for `T` that I'd get if I didn't give any explicit bounds, plus some explicit bounds".** Suppose that underscore (`_`) stood for the recursive bounds, i.e. the syntax were `T: _ + <extra bounds>`, leading to `#[derive_where(<traits to derive>; T: _ + <extra bounds>)]`, then the above diff would become ``` diff -#[derive_where(PartialEq, Eq, PartialOrd, Ord; T)] +#[derive_where(PartialEq, Eq, PartialOrd, Ord; T: _ + Sync + Send + Any)] struct Node<T, D> { ... } ``` instead :) I assume this is a common problem whenever explicit bounds for the generic parameter are specified, but I'm not sure, as this is the first time I've ever needed explicit bounds (usually I'm just using `derive_where` to ignore phantom parameters).