Link references to std in docs

[KodrAus]

Relevant API Guideline

Turn references to items in the standard library in docs into references to that item in https://doc.rust-lang.org/stable/std/, so for example:

Reference to [`std::io::Error`][IoError] in markdown docs.

[IoError]: https://doc.rust-lang.org/stable/std/io/struct.Error.html

[expenses]

I'll have a go at this!