Add derive support

[BrynCooke]

Often you just want a builder for a struct without wanting to create a constructor.

This is a natural progression for the code:

1. Struct with builder
2. Struct starts needing bespoke handling for construction.
3. Use constructor with builder.

Idea is that a derive macro could be introduced that allows derivation of a new method and builder.

[BrynCooke]

#[derive(buildstructor::Builder)]
pub struct MyStruct {
    simple: usize,
}

This will derive the following:

#[buildstructor::buildstructor]
pub impl MyStruct {
    #[builder(visibility="pub")]
    fn new(simple: usize)->MyStruct {
        Self {
          simple
        }
    }
}