Nested builder support

[ijackson]

This builds on the codegen #245 to provide the ability to sensibly nest structs with builders. With this, our code in Arti can do this:

#[derive(Clone, Builder, Debug, Eq, PartialEq, AsRef)]
#[builder(build_fn(error = "ConfigBuildError"))]
#[builder(derive(Deserialize))]
pub struct TorClientConfig {
    /// Information about the Tor network we want to connect to.
    #[builder(sub_builder)]
    tor_network: dir::NetworkConfig,

with the result that:

• TorClientConfigBuilder contains a NetworkConfigBuilder.
• Callers can do things like tor_client_config_builder.tor_network().fallback_caches(...)
• There is no need for anyone to write out the .build() calls for the sub-structures. tor_client_config_builder.build() does it.
• impl TorClientConfigBuilder { pub fn tor_network(&mut self) -> &mut NetworkConfigBuilder { ... } } can now be autogenerated rather than our current handwritten methods

sub_builder changes defaults for the contained field type and how to construct the target field, but you can still override them with the features from #245

I guess you will have some opinions about the shape of the field attribute syntax...

Thanks again for your attention!

[TedDriggs]

Can you file an issue for this proposal?

I'd like to better understand the use-case and available alternatives before reviewing a particular implementation.