Add no-std support

[danieldg]

Fixes #5.

@kesyog, please test this before I release 0.4.4 with it.

[kesyog]

Hey, thanks for looking at this so quickly. I took a quick look at the PR and left some comments around the feature flag definition.

[kesyog]

Rather than adding a no-std feature, you should add an std feature and make it a default.

From https://doc.rust-lang.org/cargo/reference/features.html#feature-unification:

If you want to optionally support no_std environments, do not use a no_std feature. Instead, use a std feature that enables std.

An example: https://github.com/matklad/once_cell/blob/master/Cargo.toml

This would allow you to make unpin dependent on std. Right now, enabling both unpin and no-std wouldn't work.

[danieldg]

This does not work: you cannot enable a dependency based on the lack of a feature. Renaming the feature to critical-section might be better, however.

[danieldg]

Right now, enabling both unpin and no-std wouldn't work.

This actually does work, but demonstrates why the name no-std is misleading: the normal types will continue to use critical-section Mutexes, and the unpin module is also available.

[kesyog]

I meant that it wouldn't work because unpin contains std types, but it looks good to me now that unpin depends on std

[kesyog]

I didn't do any functional testing but I read through the code and checked that I could build for a no-std target for a test project with:

[dependencies.async-once-cell]
  git = "https://github.com/danieldg/async-once-cell"
  branch = "no-std"
  default-features = false
  features = ["critical-section"]