Make crate `no_std` compatible

[ghost]

The current implementation of the crate is not no_std compatible, since it is not marked with the#![no_std] attribute.

This PR addresses that, as well as improving the crate in general:

• Code now relies on no_std_compat to simplify no_std/alloc/std management
• A new feature was added alloc, enabled by default, which adds what the no_std feature previously removed
• The crate is now compatible with no_std + alloc, so std is not required at all
• Touched up the Display impls a bit to make use of the write! macro

Tests were updated and one test was removed when the alloc crate is not present, as it would fail in certain repository conditions, as expected.

[kinnison]

Wow, thank you so much for this. I'm not much of a no_std user but I definitely wanted this crate to work well in that kind of environment. This includes a new-to-me crate so it's a good learning point for me.

The only CI failures are in beta/nightly clippy which is fixed on master, so I'm merging this, thank you again!