Rust 2015 edition support

[ratijas]

In the current state, this crate works with Rust 2015 only under a specific conditions: explicit extern crate core; must be added to the top of the project's main file — otherwise compiler gives a lot of weird warnings per square inch of code. This is required because PR #78 introduced ::core paths instead of ::std paths for better compatibility with #[no_std]. More on topic here: https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html.

This conflict should be either a documented 'feature', or somehow mitigated via code.