Minor: stop using std::fmt, instead used core::fmt.

[martinfrances107]

This is a coding standard, I have adopted in my own libraries, and well this is open source, so I want to start using this practice in modules that I use on a regular basis too..

It follows a recent trend in the rust community.

It is a aspiration, rather than an achievable goal - One day someone will try and run my library of an IOT device
and I will have to answer question on the lines of

What portions of the library can I still use in a "no_std" environment.

So the is a defensive change .. and to drop std::fmt and use core::fmt where possible

I hope this helps.

[gbj]

Huh, that's interesting. There is no plausible path to no_std for Leptos, but seems like good practice overall!