no_std support

Thanks for this library!

Is there any interest from the devs in `no_std` support?

We have been using `corepack` as an alternative but it is inferior to `rmp` in a number of ways unrelated to the `std` support.

I might be willing to provide a patch along the lines of,
1. adding a feature `nightly` which will allow using nightly-only features like `alloc` crate
2. selecting `serde` and `byteorder` with `default-features = false` in the `Cargo.toml` in nightly mode
3. Replacing most uses of `std` prefix with either `core` or `alloc` as appropriate