Introduce base builder type-alias

[l4l]

It's quite handy to have an alias rather than manually recreating the internal type.

fn make_builder() -> FooBuilder<((), (), (), ())> { .. }

// vs

fn make_builder() -> FooBuilder { .. }

[idanarye]

1. Why would you ever need to specifically address the base builder type? I can imagine wanting to write a function that returns the builder with some fields filled in, or accept a builder with some fields filled in, or a struct that stores the builder with some fields filled in - but these all are not the base builder. The base builder itself is a fancy unit type.
2. If you still want to do it - wouldn't it be simpler to use Default Generic Type Parameters?.

[l4l]

1. I'd like to expose the type in my API, in particular builder for http client. Apparently the other, more generic option would be adding a trait, but that perhaps quite a big change.
2. That sounds better, try to look at this.