New Guide: crates and modules

Also fixes a couple of filesystem references that I forgot to change. :sweat_smile: 
How about: "However, code (be it an executable program or a library routine) usually employs functionality provided by other libraries; to support this, crates may also depend on other crates.."

Maybe a bit more florid than the other text in the guide -- the main point is to unify "executables and libraries" into one concept: "code."