Add feature 'core_collections'

[lexxvir]

Hi,

Unfortunately currently I tied to certain version of rustc. In this version collections crate is not deprecated yet. I added feature that allows to use such old rustc versions.

[rudib]

I'm not so thrilled about merging this into the master branch. Then we'd also need to properly build the test suit against an older Rust nightly, which would also require changes (conditional compilation) for the memory allocator implementation and other changes. The latest nightly will always be the target for packaged releases until Rust stabilizes the alloc feature.

If you find yourself blocked with an older Rust release, I propose to you to just use a Git reference for your fork in Cargo.toml, like so:

freertos_rs = { git = "https://github.com/lexxvir/freertos.rs" }