Add type-driven generic data access to std

[nrc]

This RFC proposes adding a provide_any module to the core library. The module provides a generic API for objects to provide type-based access to data. (Whereas the any module provides type-driven downcasting, the proposed module integrates the downcasting into data access to provide a safer and more ergonomic API).

By using the proposed API, an object can have functionality like:

let s: String = object.request();
let s = object.request_field::<str>(); // s: &str
let x = object.request_field::<SpecificContext>(); // x: &SpecificContext