Struct properties

I'd like to have syntax for defining named "properties" on a struct. A property would consist of a name, a type (which may use in-scope generics), an optional get function (taking void and returning the field type), and an optional set function. (The name and the get/set function can each independently have "pub" visibility or not.) A property will not have any storage allocated for it in the struct layout; instead, attempts to get or set the property will become calls to the get or set function. A property with no get or set function specified will produce a compile-time error on an attempt to get or set it, respectively.

The get function would take no arguments and return a value of the field type, and the set function would accept a value of the field type and return nothing. In both cases, it would also work to accept/return a reference with appropriate lifetime.

In addition to the convenient syntax and encapsulation, this would then support several useful cases. Most notably, it would support migrating an existing field to a property, removing it from the structure layout while maintaining compatibility with previous code that accessed it via field syntax.

Start by reviewing issue #419 and the repository's RFC process, since the request describes a proposed language feature rather than a named implementation task. Define the property syntax, visibility and generic rules, getter/setter behavior, references and lifetimes, layout effects, and compatibility expectations; done means those design questions are resolved in an accepted RFC.