Fixes #67

This PR adds support for the PostBuild trait, which allows users to perform custom validation when the .build() method is called. The validation runs as the last step of the build method.

An example:

use typed_builder::{PostBuild, TypedBuilder};

#[derive(Debug, PartialEq, TypedBuilder)]
#[builder(postbuild)]
struct Foo {
    x: i32,
    y: i32,
}

impl PostBuild for Foo {
    type Output = Result<Self, String>;

    fn postbuild(self) -> Self::Output {
        if self.x >= 5 {
            return Err("x too high - must be below or 5".into());
        }

        Ok(self)
    }
}

fn main() {
    let foo = Foo::builder().x(1).y(2).build().unwrap();
    assert_eq!(foo, Foo { x: 1, y: 2 });

    // Fails to validate during runtime
    // let foo = Foo::builder().x(5).y(6).build().unwrap();
    // assert_eq!(foo, Foo { x: 5, y: 6 });
}

When the user provides #[builder(postbuild)], the macro won't implement a default PostBuild impl. Instead, the user has to implement this trait manually. This allows user to implement custom validation logic.

Note: Some test cases are commented out, as the initial draft of this feature breaks these test cases. Follow-up commits will fix those issues. As such, the PR is a draft. When the test cases are fixed and the code is polished, it is marked as "Ready to review".