Add support for allocator_api on stable

[jess-sol]

This allows the allocator api to be used in Rust stable. The allocator-api2 crate also optionally implements serde support for their re-implementations of Vec, Box, etc. Which can be really helpful when trying to serialize/deserialize data that's using a custom allocator.

In order not to break things, if the allocator feature flag is set, it'll take precedence. The allocator_api2 crate will only be used if default_features is false and allocator_api2 is added.

[SFBdragon]

Looks good! Thanks for the PR. I'm aiming to merge this soon and publish a new version a bit later today once I have time to add the new feature configuration to the testing scripts and making sure everything is in order.

The features configuration looks congruent with the rest at a glance, thanks. [I don't like the current default feature configuration (unstable by default, which made more sense when I was only targeting systems/embedded use cases). I'll change it if I manage to justify a V5.]

[SFBdragon]

Alright, besides a version bump I think everything is in order now. Sorry about all the noise.

The new stable Rust example needed to be put in its own package due to the nightly-by-default config, and I felt that it was a good opportunity to turn the repo into a proper workspace.

Once again, thanks for the contribution!