Always compile as a no_std crate

[TethysSvensson]

This PR turns changes the std feature flag to do nothing, and always compiles as a no_std crate.

It also enables all code to compile and run with --no-default-features and with --all-features. Before this commit, running e.g. cargo test --no-default-features would fail both on stable and nightly.

Note that this introduces a bit of noise in the output when running cargo test. It looks like this:

error: no global memory allocator found but one is required [...]

Now obviously this looks like it's an error, but it is actually safe to ignore does not actually affect the tests at all. See this comment for more context.

Note also, that nightly clippy gives two additional warnings about doctests in raw_vec.rs. I chose to ignore these, as the tests do not actually work as far as I can tell, are not actually run (they are ignore) and are they part of any public interface.

I promise this is the last PR I will bother you with for now. 😉

[fitzgen]

Thanks!