Add Rust/-Lang Solutions

[dan-fritchman]

Proposing adding the Rust-lang Solutions available here as a submodule:
https://github.com/dan-fritchman/CtCI-6th-Edition-Rust.git

They include all chapters and problems, and I believe show a number of idiomatic Rust-lang approaches to them.
Hope they prove helpful. This was submitted as PR #227 (some time ago; sorry I lost track of it.)

Per that PR's commentary, I have read the requests here:
https://github.com/careercup/CtCI-6th-Edition#adding-a-new-language
This issue intends to fulfill its second request.