Crubit rs_std library

This directory contains the rs_std C++ library that provides the following APIs:

• Manually authored APIs that help work with Rust builtin types. For example, rs_std::rs_char represents Rust's char type (a separate type from C++'s char32_t is needed to detect certain invalid bit patterns that result in Undefined Behavior in Rust; additionally char32_t takes at least 32 bits, rather than exactly 32 bits).
• (Not yet implemented) Automatically generated C++ bindings for Rust standard library.