Finally I completed the `no_std` PR.

The nostr crate now can be used in embedded devices 👀

Thanks to nostr:npub1vjxrxzr5epxxf5qzxeachshvafhcj5utct5dlw60qx7ayvq95mhsypdl8u for the help

https://github.com/rust-nostr/nostr/tree/master/crates%2Fnostr%2Fexamples%2Fembedded