[FEATURE REQUEST] no std support

[Lohann]

Would be very helpful for me if this library support no_std environments, this would make this crate usable in embedded hardware such as the surveillance cameras, this also improves the performance, once doing heap allocation for encode/decode every message is quite expensive.

If the maintainer think this could be an good idea, I can help on remove the std dependency for this project, at first glance the only dependency which requires STD is the url library, that can be replaced by fluent-uri.

Another plus would be make alloc optional too and only expose MessageRef and other ref only structs when alloc is disabled.