Add an option to make struct fields public

[sfackler]

Before this PR

Like in Conjure-Java, we made generated objects immutable with a builder API. This makes the structs a bit more annoying to work with, since you can't directly mutate them and can't move out of them. Since Rust has a robust ownership model, the need for immutable types is not as large as it is in Java.

After this PR

==COMMIT_MSG==
Added a publicFields codegen option that replaces struct accessor methods with public fields.
==COMMIT_MSG==

The builder types are still around, and since we mark the structs #[non_exhaustive], are the only way to construct objects. This makes it a bit easier to add new fields to objects without breaking builds, and are generally a bit easier to work with since the builder methods handle things like type conversions.

To preserve backwards compatibility, the option is disabled by default, but I think we may want to make this how we do things moving forward.

Possible downsides?

We're now publicly exposing the fact that struct and union fields are boxed internally. We do this to both keep the shallow struct size from getting too enormous and to avoid infinite sizes for recursive APIs.

Closes #304

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Description

Added a publicFields codegen option that replaces struct accessor methods with public fields.

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✨ Features

• Added a publicFields codegen option that replaces struct accessor methods with public fields. (#522)