Feature Request: Builder closures as args (nested/sub builders)

[polarathene]

I was curious about a better approach to this awkward separate builder example:

// fn new(pair: Pair<String, String>) -> Self
builder().pair(
    Pair::builder().x("hello").y("world").build()
).build();

The DX could be improved perhaps by supporting closures to abstract another builders entry (T::builder()) + exit (.build()) calls?:

// Separate builder for `Pair`:
// fn new_pair(x: String, y: String) -> Pair<String, String>

// type signature for `pair` probably needs to change for builders that leverage it?:
// fn new(pair: Pair<String, String>, another_param: Option<bool>) -> Self
builder().pair(|p| {
    p.x("hello").y("world")
})

Nesting builders with sub-builders would seem a bit nicer with that approach if it's viable?

---

This is a bit related to issues:

• TryInto<T> support + Into<T> with scalars #186
• docs: Implicit multi-arg support for builder params? #187 (shows existing alternatives via tuple or separate args, also requests multi-arg support)