Add support for no_std

[daxpedda]

This adds support to no_std environments by using default-features = false and adding features = ["alloc"] to the serde dependency. To preserve the MSRV, I added an std crate feature and enabled it by default, as unfortunately the Error trait hadn't been moved to core at v1.56 yet. So I'm unsure if there are any guidelines if this counts as a breaking change, but I believe not.

I also adjusted the CI to test without default features and with. Let me know if you want me to add that matrix to cargo doc as well.

[daxpedda]

Apparently serdes v1.69 support for alloc was broken and fixed in serde-rs/serde#1336. So I had to bump the serde dependency to v1.70.