no_std compatibility

[archseer]

We currently rely on some parts of std, but having a no_std build would be interesting for embedded devices and WebAssembly. Note that a lot of the std stuff exists in core, so we could get far with just conditional imports.

https://github.com/serde-rs/serde/blob/bb99b31eb0a55393101f9c80cd959b3a739ad70f/serde/src/lib.rs#L141-L143

• Fix all the imports to import core:: + alloc versions on the no_std feature flag.
• Make sure all the dependencies are no_std capable.
• Move any OS specific code to a sys subfolder.
• Abstract the tokio runtime so we can switch to juliex or any other no_std capable executor. Look into fuchsia's executor as an example.