Handle deprecated fields

[mauricelam]

If I have a deprecated field in my struct, the generated TypedBuilder has two unexpected behavior:

1. It complains about the used of deprecated fields in the definition, but adding #[allow(deprecated)] on the struct doesn't help (because it's used in a separate, generated block?)
2. The generated setter is not marked as deprecated, so users of the builder won't know that they are setting a deprecated field.

Example:

#[allow(deprecated)]
#[derive(Debug, TypedBuilder)]
pub struct StringConfig {
    #[deprecated(note="Do not use.")]
    #[builder(default = true)]
    pub save: bool,
}

[mo8it]

That should be easy. We need to add #[allow(deprecated)] to the last line of build() where in this example StringConfig { save }.into() is called.

Why is there an .into() btw?

Then we need to add the same deprecation note of the field save to the method save.

Do we want to throw a compilation error if the default attribute is not set?

@idanarye Can I give it a try for a PR?

[idanarye]

@mo8it Go for it.

The into, BTW, is because of #80. It did cause a problem for #84, so I'll have to make it conditional on whether or not #[builder(build_method(into))] is used, but there are other things blocking #84 so I leave it in its branch for now.