Support free functions

[BrynCooke]

Maybe we can support functions that are not on an impl

Alternative 1:

Directly on fn outside of impl.

#[builder(entry="foo", exit="call")
fn do_foo(a: usize) {
...
}

Alternative 2:

Still require an impl. Allow derivation of proxy method:

#[buildstructor::buildstructor]
impl Foo {
  #[builder(entry="foo", exit="call", proxy=true)
  fn do_foo(a: usize) {
  ...
  }
}

//Derived
fn foo() {
  Foo::foo()
}

[CGMossa]

While using bevy_ecs, I want to add entities that has different sets of components attached to them.
This means that what I'd ideally want, is a way to construct a builder, around a function, but the resulting struct should atleast derive(Bundle).

[BrynCooke]

@CGMossa Could you elaborate a bit more?
The final struct that a buildstructor builder spits out is not derived, it is the result of a constructor being called. Thus you can put derives on that struct no problem.
Probably some code examples would help me understand.